The Mendelssohn Choir of Pittsburgh joined forces yesterday with soloists from the Pittsburgh Opera Center and a contingent of the Pittsburgh Opera Orchestra for a pleasant afternoon of "Opera's Greatest Choral Hits" in Carnegie Music Hall, Oakland. The joint venture previewed the work of the choir's conductor-designate Betsy Burleigh, who takes over as music director July 1, and it introduced San Francisco Opera chorus director Ian Robertson, who conducted the present event.

Following a fire scare that necessitated a pre-performance evacuation of the hall, this was a program of operatic chestnuts: the Anvil Chorus from Verdi's "Il Trovatore," the Sextet from Donizetti's "Lucia di Lammermoor," the Bridal Chorus from Wagner's "Lohengrin" and the Triumphal March from Verdi's "Aida."

Efficiently prepared by the assistant conductor Katherine Mueller, The Mendelssohn Choir sang cleanly, with clear textures and a low-vibrato purity of line. As choral singing, the Mendelssohn's work was generally praiseworthy. The ensemble does not, however, sound like a genuine opera chorus. At times I had the feeling they were still singing "Messiah" -- only with the notes changed.

For this very reason, the priestly "O Isis und Osiris" chorus from Mozart's "The Magic Flute" was one of the afternoon's most satisfying moments. On the other hand, with 90-plus voices in tow, there should have been more sound produced for the stentorian Triumphal Chorus. In that number, in fact, it was the trumpets in the orchestra who dominated in tone and volume.

Elsewhere the Opera Orchestra played unevenly: rough and ready in the Overture to Rossini's "Italian Girl in Algiers" and the Prelude to Act 3 of Wagner's "Lohengrin," yet very appealing in lyric segments -- high on that list, the Barcarolle from Offenbach's "The Tales of Hoffmann" and the Humming Chorus from Puccini's "Madama Butterfly."

Robertson's conducting was lucid and precise, despite some passages that could have used more rehearsal time. A chorus from Mozart's "Idomeneo" began awkwardly but then jelled and benefited midway from the melting solo lines of soprano NaGuanda Nobles. And Robertson made the most of the popular chorus of Hebrew slaves from Verdi's "Nabucco." He was also an amiable narrator, explaining the dramatic situations of each extract clearly and concisely in advance of each group on the program.

Among the Opera Center soloists, soprano Nobles was clearly the star. Her creamy voice has a recognizable timbre, with a distinctive edge that carries. She just doesn't sound like any other soprano. She has a solid technique, and she phrases with maturity and eloquence. Her rendition of Madama Butterfly's "Un bel di" was something quite special.

Equally a highlight was the gorgeous tenor-baritone duet from Bizet's "The Pearl Fishers" -- vividly brought to life by Arthur Espirito and Craig Verm. Jennifer Holloway's rich mezzo sounded so lovely (and blended beautifully in the "Hoffmann" duet with her fellow Opera Center mezzo Karen Mushegain) that one might have wished a more prominent part for her in this program. If some of the other apprentice artists seemed not quite ready for the heavier repertory involved, they still showed, for the most part, promise and enthusiasm.

And speaking of enthusiasm, the ubiquitous Drinking Song from Verdi's "La Traviata" brought the proceedings to a conclusion in which everyone -- on both sides of the stage -- seemed to be having a grand old time.