A new beginning of any kind is inevitably fraught with excitement. When it is one that is also brimming with promise and adventure, as is the case with the new start the Pennsylvania Ballet is making this season, it becomes doubly stimulating. At Philadelphia's 1,800-seat Shubert Theatre Thursday night, the troupe launched its most ambitious effort since the installation, last September, of Robert Weiss as artistic director. The company's inaugural performances under Weiss' direction actually took place early this month, but Thursday night's program was a blockbuster--with a world premiere and three company premieres--that gave a first comprehensive inkling of where the troupe is headed.

Reorganization also brings with it a certain degree of instability, and at least initially, a sense of experiment and testing of waters. When the Pennsylvania Ballet was last in Washington, at the Kennedy Center Opera House last May while the company was still led by Benjamin Harkarvy, the dancing, staging and repertory seemed to have reached a new peak of excellence, just at a moment when the troupe's financial status, ironically, was in dire crisis. On Thursday the company, largely unchanged in personnel but attempting four new works out of five, had the look of a group not quite certain of its identity and not altogether fused in purpose, even though the performances were ablaze with fervor. The evening's rewards, however--the general aura of enthusiastic renewal and the specific achievements of the new repertory--suggested that present defects are probably transitory, and that the company is undergoing a healthy and vital rebirth.

To recapitulate briefly, at the start of last year the company reshuffled its administration in an effort to cope with a $2.7 million deficit. Barbara Weisberger, the George Balanchine prote'ge'e who had founded the troupe in 1963, resigned in February; in July, Harkarvy--artistic director for 10 years--announced he wouldn't return. At the same time, Weiss, a 33-year-old former principal of the New York City Ballet, was appointed in his stead, and NYCB's Peter Martins, widely expected to be Balanchine's successor, was named to the new post of artistic adviser. Since then, $1.7 million of the deficit has been erased, and seven dancers, including former NYCB member Debra Austin, have been hired to replace others who left.

The high point of Thursday's program was the world premiere of "XVIII Symphonic Etudes," to Robert Schumann's piano score of that title, by 34-year old Richard Tanner, another NYCB alumnus, sporadically active as a choreographer. The new 35-minute opus--a lyrical, romantically effusive "piano ballet" (pianist Martha Koeneman was on stage) for seven couples, one at a time and in various groupings--is indicative of a formidable creative talent. In this work the step vocabulary is almost purely classical, though Tanner has ventured a wide variety of modes elsewhere. What's original here is the enticing combination of traditional movements, and their exceptionally sensitive musical disposition. The ballet also looked tailor-made for its Pennsylvania Ballet cast, a cue that Tanner has a fine grasp of dancers' individual needs and gifts.

It was also gratifying to see the company's Tamara Hadley and Jeffrey Gribler so neatly dispatching the thorny difficulties--both technical and expressive--of Peter Martins' "Calcium Light Night," the choreographer's first and in many ways most impressive ballet, created in 1977 to a spiky, witty Charles Ives anthology. Though the Pennsylvania dancers, understandably, couldn't muster all the electric incisiveness the ballet demands, coaching by Martins himself and Heather Watts of the original NYCB cast obviously paid off handsomely.

An equal achievement for the dancers was Paul Taylor's "Arden Court," another company premiere. The Pennsylvanians gave an expectedly balletic look to Taylor's brawnily curvaceous modern dance idiom, and had their troubles with the work's lightning directional changes and muscle-twisting shapes. But the energy, eloquence and formal clarity they attained bespoke a splendid understanding of Taylor's aims. The performance of Balanchine's "Tchaikovsky Pas de Deux" also fell short of perfection but had its strengths as well--the flashily unkempt Rumanian newcomer Marin Boieru was out of his depth, but the exquisitely poetic Melissa Podcasy--the company's most fully matured principal artist--was shown off to alluring advantage.

The one disappointment was the company premiere of director Weiss' own weakly conceived and executed "Gli Uccelli ("The Birds"), to Respighi's tacky, neo-rococo score. Still to come in the way of new repertoire during a season that extends through June is the world premiere of Martins' production of "La Sylphide" and a new ballet by Lynne Taylor-Corbett. The signs are that the Pennsylvania Ballet is off to a flying future.