The past meets the future in two plays running on Pittsburgh stages. While they both give audiences scenes of exceptional theater, their muddled endings don't deliver on the promises the plays make early on.

Blame it on the playwrights August Wilson and Tom Stoppard.

In "Seven Guitars," the fifth of Wilson's 10-play cycle and staged by Pittsburgh Playwrights Theatre Company, the American dream is thwarted by an unreasonable allegiance to a mythical past.

Presented by Pittsburgh Irish and Classical Theatre, Stoppard's "Rock 'n' Roll" is a critical and popular success in a long career of triumphs. It demands that we consider the ideals of communism while we gather souvenirs from the rubble of the Berlin Wall.

Yet, despite the heady intellectual drive of Stoppard's writing, he resorts to a convenient conclusion slapped together from Jane Austen novels rather than Karl Marx's theories. Considering Marx's writing style, that probably makes sense, but not satisfying theater.

Clearly from the titles, music is the central motif of both. Wilson steeped himself in the sounds and meaning of the blues and wrote his plays with a rhythm and poignancy to match.

Stoppard grabbed the energy and insolence of rock music, turning it into a symbol of personal freedom in a climate of repression in his native Czechoslovakia.

Legendary musicians play muses -- Buddy Bolden for Wilson, Syd Barrett for Stoppard. Cornet player Bolden, who died in 1931, pioneered jazz playing in New Orleans. Barrett was an original Pink Floyd member, who left the band after a breakdown in 1968.

Both plays open with songs. Louise, resident of a Hill District tenement, coos a bawdy blues ballad. Barrett perches on a garden wall in Cambridge, England, singing "Golden Hair" to a golden-haired 16-year-old.

From the dirt backyard in 1948 Pittsburgh to the greenery of an English garden in 1968, the plays are poised on the edge of change. America is beginning the slow dismantling of segregation -- the armed forces were integrated that year -- while "the Prague Spring" in Czechoslovakia anticipated the collapse of Soviet domination two decades later.

The play's characters both anticipate and resist the changes, creating the tug and pull that charge the works with tension.

The sets of "Seven Guitars" and "Rock 'n' Roll" play major roles by making imaginative use of the cramped quarters where the works are staged.

Mark Clayton Southers, who also directed the Wilson play, re-creates in the tight space of 542 Penn Ave., Downtown, the neglected, down-at-mouth feel of the Hill a few years away from its widespread destruction for the Civic Arena. From its dirt floor to its sagging porch railing, the set mirrors the poverty of its inhabitants.

Faced with a play that moved ahead 12 years and jumped back and forth from Cambridge to Prague, designer Narelle Sissons elegantly combined 18 projection screens and a widened stage at the Heymann Theatre to allow the production to flow smoothly.

PICT's "Rock 'n' Roll" is both a triumph of technology, using still and moving pictures in concert with rock music, and solid dramatic achievement by a cast of polished, confident actors.

Under the direction of PICT artistic director Andrew Paul and in the hands of veterans Helena Ruoti, Sam Tsoutsouvas, Sam Redford and Simon Bradbury, discussions of dialectical materialism and Sapphic poetic meter seldom sounded so entertaining. Stoppard's writing demands intelligence and preciseness. That quartet manages it all quite nicely.

"Rock 'n' Roll" depends on more than political debate. In the rocky marriage of the academics Eleanor and Max, Stoppard creates a powerful emotional conflict between the demands for love and for recognition.

Ruoti inhabits the cancer-ravaged body of the classics scholar Eleanor with a fierce determination to stand as an equal to the stubborn anachronism of Max, a hard-line Marxist and argumentative bully, played by Tsoutsouvas with the proper bluster.

Near the end of the first act, Ruoti erupts in a moving scene of pain and despair. "I want your grieving soul or nothing," she tells Max who's been lecturing everybody about materialism, Soviet style. Concluding nearly 90 minutes of intellectual repartee, it's a shocker, made even more so by Ruoti's intensity.

The need for love also leads to an intense and moving scene in the first act of "Seven Guitars." Here, Vera, a young woman jilted by performer Floyd "Schoolboy" Barton, throws his casual suggestion of reunion back in his face:

"... I looked up and you was back here after I had given you up. After I had walked through an empty house for a year and a half looking for you. After I would lay myself out on that bed and search my body for your fingerprints..."

Pittsburgh newcomer Ericka Ratcliff invests the role of Vera with the powerful pride of a person scorned. Like Ruoti, she and the audience are left shaken by the force of her wounded soul crying in pain.

Played by Homestead native Montae Russell, a Wilson play veteran, Schoolboy is the future of "Seven Guitars." Down on his luck, his electric guitar trapped in a pawn shop, he is riding high when his recording is getting airplay in Pittsburgh.

His sound anticipates the amplified music of the 1950s, but Schoolboy doesn't have the cash for a Chicago recording trip to claim the success he so desperately craves.

His opposite is King Hedley (Wali Jamal), another tenement boarder who is sinking into the craziness of frustration with America's discrimination. At 59, he clings to the dream of his father's debt repaid by Buddy Bolden so he can escape white culture and live like, well, a king.

Where Barton courts his white manager, Hedley waves his machete and says, "Now Hedley ready for the white man when he come to take me away." The opposites are destined to clash in the play's climax.

(He is the inspiration for Wilson's 1999 "King Hedley II" that debuted at the Pittsburgh Public Theater.)

Jan is a young Czech student in England working with Max in '68, but more in love with rock 'n' roll than the dictatorship of the proletariat. As Stoppard tells us, it's Jan's political apathy that is the true threat to communist dogma and major irritant to hardliner Max.

The English actor Redford, part of the fine cast of "The Seafarer" earlier this year at City Theatre, interprets Jan as a bright, yet naive young man eager to please his mentor and later, his own country's secret police, done in a nasty turn by Bradbury. Jan learns the hard way that apathy isn't possible during a time of political upheaval.

Both plays raise serious social and political issues, issues that challenge the private lives of their characters and the hearts and minds of their audiences.

This is daunting theater, performed with professional elan and energy by the Playwrights and PICT. The disappointment comes when we discover that both playwrights have lost their nerve by the last scene.

"Rock 'n' Roll" declines into a pleasant domestic comedy; "Seven Guitars" glosses over its hopelessness with religious cliches.

Don't blame the theater companies, though. PICT and Playwrights have met the demands of these challenging plays with spirited performances that deserve our attention.