WARM-up exercises," said Donald Sadler. "Everyone at the barre, please."

The cast of the opera "Wiener Blut" obediently moved to a ballet-style metal bar in the Kennedy Center Terrace Theater's rehearsal room. "Inhale and up," said Sadler, "one and two and three and four . . ." all the way up to eight. The cast inhaled, slowly, in unison, one hand gripping the barre while the other rose in time with their breathing until their lungs were full and their arms were pointing straight up.

"Now," said the noted choreographer, "exhale and relax." Eight people exhaled simultaneously and collapsed to the floor like very limp but very graceful rag dolls.

An unusual workout for a group of opera singers -- particularly for a production aimed at the Terrace Theater, whose stage seems hardly large enough for a really vigorous pirouette. But this opera -- a romantic comedy of mistaken identity set at the Congress of Vienna in 1815, and full of dance sequenes -- has a special role in The Washington Opera's season. General director Martin Feinstein explained: "We chose 'Wiener Blut' because we felt it would be a contrast to the rest of this season's repertoire, which ranges from baroque to contemporary, Italian grand opera to very intimate opera. Dramaturg Frank Rizzo suggested 'Wiener Blut,' which he has seen in Vienna, because it has small forces, unlike 'Die Fledermaus,' and can fit both our stage and our budget.It has all of Strauss' great hit tunes, and the music literally dances."

That is why, a week before the scheduled Christmas Eve opening, the personnel of the gala holiday production looked remarkably like a ballet troupe warming up. The women were in leotards -- although Karen Hunt (who plays a countess) wore a long petticoat to simulate the lavish Empire-style costume she will be wearing onstage. Janice Hall, who plays a dancer, was costumed as one, and so was Sunny Joy Langton, who plays a seamstress with ambitions as a dancer. Drawings of their costumes were pinned to bulletin boards in the rehearsal room, as well as sketches of stage settings and pictures from the Napoleonic era in which "Wiener Blut" (literally, "Viennse Blood," but meaning spirit or soul) is set.

"This is an operetta, not a ballet, isn't it?" someone asked tenor Russ Thacker, who plays a valet and does a lot of dancing. "Yes," he said, "but when you're playing this much three-four, you just naturally want to dance." Thacker, who has a lot of Broadway experience, is already a seasoned dancer. "It's nothing new to me," he said, "but I think it may be a strange experience for some of the people who have specialized in opera."

So it is, confirmed tenor Warren Ellsworth, who plays a count in this production and will be back in a few months to sing Pinkerton in "Madame Butterfly." He has no professional dancing experience and tends to laugh when the subject comes up. "The countless and I have to waltz in several different ways," he says, "and that's about the extent of our dancing -- otherwise, we would be hard-pressed. I can only speak for myself, but I think we are lucky to have Donald Sadler, who has had so much success in classical ballet and on Broadway, to make people like me look actually pretty good. He has brought us along with great patience and love."

The music for "Wiener Blut" is by Johann Strauss II -- the last Strauss operetta of his long lifetime, and one comparable to his operetta masterpiece, "Die Fledermaus," in the number and quality of its melodies. One point that the Washington Opera production will establish firmly is that Strauss was a specialist in dance music. But when he wrote the dance tunes in this show, he didn't know they would be enshrined for posterity in an operetta.

The year was 1899 -- the end of an era and the last year in the life of Strauss, who had contributed a special flavor to that era . When the 73-year-old composer was approached for a new operetta, he felt too old and tired for such a large assignment, but he did agree to help by going through his earlier works (nearly 500 of them) and picking music that could be adapted for the stage by a younger collaborator: composer-conductor Adolf Mueller Jr.

The nucleus of the last Strauss operetta was a waltz, "Wiener Blut" (op. 354), which he had composed a quarter-century earlier -- at about the same time as "Die Fledermaus." Many of the other items selected were of the same vintage, and a good reason for an abundance of dance music was provided by making the whole second act a ballroom scene. By the time the work was ready for the stage, Strauss had died of pneumonia and the operetta became, in a sense, his epitaph -- an appropriate one, since it includes somes of his finest melodic inventions.

The melodies, harmonies and orchestration in this production are entrusted to Hugh Wolff, a young conductor who has been working with the National Symphony for some time but will be making his professional operatic debut in "Wiener Blut." At first, he recalls, he hesitated. "Martin Feinstein offered me this assignment last spring, when I substituted for Antal Dorati with the National Symphony," Wolff said, "and I asked him, 'Can I change the piece or is it this or nothing?' 'It's this or nothing,' he told me, so I said, 'All right.'" Wolff hesitated not because he looks down on the music but because he knows how hard it is to conduct Strauss.

"You hear people saying, 'Let's do an operetta or a Braodway show because it's easier than opera,' but that's just not true," Wolff said. "I had a conducting teacher with whom I studied for many summers, and he always included Johann Strauss in his course. He said it was the toughest repertoire to conduct -- beyond 'Till Eulenspiegel' and beyond 'The Rite of Spring.' And if you could do it properly you had earned your spurs as a conductor."