When Pennsylvania Ballet dancers are performing well and having a good time, their joy is infectious.

The opposite is also true: When they're tired, the steps are especially difficult, or someone has made a mistake, the whole performance suffers.

The company premiered two ballets Wednesday night in its Modern Masters program. One was exciting and fun. The other should have been as well, but it was too much of a struggle to be a success.

Lambarena, which Val Caniparoli choreographed in 1995 for the San Francisco Ballet, was an ideal piece for the Pennsylvania Ballet. An African-inspired work, it offers enough range of motion and emotion to show the dancers in the best light. And because precision was not the goal here, the dancers were able to fearlessly pour themselves into it.

The men's poses, stretches and turns in attitude position at times evoked gazelles, giraffes and other creatures of the African veldt.

The women danced a combination of pointe work and African movements - contractions, swaying hips and scooping arm motions. Their full-skirted, brightly colored dresses were swept up into the action.

Two standouts in the excellent cast were the company's only two African American dancers: Heidi Cruz-Austin, who was equally engaging in her shimmying solos and her classical pas de deux, and Jermel Johnson, who threw himself into an emotional solo of slow stretches and sharp turns.

Lambarena is set to an unusual but effective score: traditional African songs in some sections, music by Johann Sebastian Bach in others, all overlaid with percussion sounds and occasional bird calls.

The company had less success with In the Upper Room, which Twyla Tharp created for her own company in 1986.

It's a highly aerobic ballet with some dancers on pointe and others in sneakers, all dressed in black-and-white striped pajamas with lots of touches of red - pointe shoes, leotards, socks and short skirts.

Two problems marred the performance early on: A partnering mistake, with the female half of the couple unable to hold her arabesque or stay on pointe; and a fog machine that worked overtime, obscuring dancers far too much and sending clouds wafting into the audience.

There were some lively sections and individual performances, but many of the dancers were visibly weary before the ballet ended, and their movements looked heavy.

Contact writer Ellen Dunkel at edunkel@phillynews.com.

Pennsylvania Ballet will repeat the Modern Masters program today through Sunday at the Merriam Theater, 250 S. Broad St. Information: 215-336-2000.