FOR MORE reasons than one, American Ballet Theatre seems for the moment to be drifting rudderless.

The company appears to have weathered the departure of Mikhail Baryshnikov, and the subsequent unexpected retirement of Ivan Nagy-the loss, almost at one stroke, of two to its most highly valued male principals-very sucessfully. The box office is holding up well everywhere, so far; the three-week Kennedy Center engagement, which ends tonight, drew sell-out crowds most of the run.

On the long-range fiscal front, the picture looks equally benign. A $1-million challenge grant from the National Endowment for the arts last year, coupled with an administrative reshuffling and a general tightening of the corporate ship under new executive director Herman Krawitz, appear to assure a much less crisis-ridden future for ABT.

But a number of factors currently conspire to undermine stability on the artistic side. First among these is the imminent withdrawal of artistic director Lucia Chase, who this past January announced her retirement-effective as of fall 1980-after 35 years of courgeous, often controversial leadership. The prospect of the step-down has given the company something of the aspect of a lame-duck operation.

It also has goaded the gossip adoring inner circle of the ballet world-there are those who take more pleasure in a juicy rumor than in the most sublime of performances-into an orgy of speculation about successors to Chase, such as: Rudolf Nureyev will seize the opportunity to finally settle down and rule a roost of his own; or Mikhail Baryshnikov, alleged to be increasingly restive over his New York City Ballet status, will return to his erstwhile fold and gradually relinquish dancing altogether.

More concretely, the troupe has rarely, in recent years, had such a trimmed look, particularly at the top. Besides Baryshnikov and Nagy, principals who have left in the past couple of years include Ted Kivitt, Karena Brock, William Carter, Gayle Young and Charles Ward.

In addition, Clark Tippet is inactive, and Sallie Wilson seems to have receded into virtual retirement. Just slightly lower in the ranks, soloists George de la Pena and Leslie Browne are off on extended leave making the Nijinsky film that Herbert Ross (of "Turning Point" fame) is directing. All these goings, despite some notable comings, have put the secondary echelons of the company's dancers into a new framework of challenge and duress, and added as well to the strains on a repertory tailored to an earlier phase in the company's history.

Nevertheless, the company's strongest feature right now would appear to be its exciting cadre of dancers. The arrival of Anthony Dowell, on indefinite loan, so to speak, from England's Royal Ballet, had given the troupe a male presence of unquestioned nobility, authority and range, a range which he extends into new realms of sensibility with every appearance on stage.

Among the other male principals, Fernando Bujones remains an invincible virtuoso and rather erratic in other aspects of performance, though he's made definite strides towards artistic maturity in recent seasons. John Meehan, looking leaner and handsomer than ever this season, also has been dancing better, and he's shown us more of his formidable dramatic gifts in such ballets as "Miss Julie" and "Fall River."

Young Patrick Bissell, who becomes a company principal as of tomorrow, has still a lot of raw edges, but also clearly the potential for greatness. Kevin McKenzie, the Washington-trained dancer late of the Joffrey Ballet, has been taken on by ABT as a soloist, but already in this Washington engagement danced with the assurance and panache of a principal.

Natalia Makarova appears to have attained a state of inner serenity that is giving to her always stylistically impeccable dancing newly resonant depths. Some of this may be ascribable to domestic harmonies, some to her effusive partnership with Dowell; mostly, though, it's probably due, as she herself claims, to a natural growth of the spirit. Cynthia Gregory, as brilliant a technician and artist as ever, is also characteristically unpredictable still-like the very different but equally individual Martine van Hamel, she has nights when she is supreme, and nights that are inexplicably lack-luster.

The most puzzling and worrisome case is Gelsey Kirkland. This most extraordinary artist, unrivalled in fluency and emotional conviction, has been dancing magnificently, but her appearances have been alarmingly infrequent.

Further into the ranks, there are a number of exceptionally notable newcomers. Lise Houlton, who joined ABT last year as a corps dancer but who has already been assigned several substantial roles, exhibits a delicacy, passion and refinement which claim instant attention-if she's being groomed for higher things, there seems good cause. Also relatively new to the corps is Washington-born David Loring, whose line, placement and presence bespeak uncommon promise-he too is getting some individual, and deservedly so, exposure in a performance this weekend of Tudor's "Triller in the Fields."

The company also has wisely been using this "interim" period to give a number of gifted dancers of still indeterminable future additional chances to test their mettle-among the most impressive of these are Gregory Osborne and Victor Barbee, both about to be promoted to soloist rank; Kristine Elliott and Danilo Radojevic, who are already soloists; and Christine Spizzo and Johan Renvall of the corps de ballet.

It's the repertory that is the most troubling and dubious aspect of ABT at present, and particularly the leading edge of new additions. Over the past two Kennedy Center visits this year and last, the company in its role as a "living museum" of ballet tradition has continued to bring us, with every justification, a healthy sampling of full-length ballets, old and new, as well as briefer classical staples and emblems of ABT's rich heritage of works by the likes of Ashton, Tudor, Balanchine, De Mille and Robbins. Showy pas de deux for the sheer sake of exhibitionism seem to have been discarded altogether-certainly a healthy development and a sign of the increasingly discriminating taste of the ballet public.

But of the six ballets added to the repertory during the just-ending Kennedy Center series-Glen Tetley's "Contredances" and "Pierrot Lunaire"; John Neumeier's "Desir"; Ben Stevenson's "Three Preludes"; John Cranko's "Pas de Deux Holberg"; and Birgit Cullberg's "Miss Julie"-only the first was genuinely "new," in the sense of having been newly created, and specifically for ABT. Moreover, of these choreographers, the only Americans-Tetley and Neumeier (Stevenson resides in Houston, but he's English by birth and stylistic persuasion) have long lived abroad.

Where in the venturesome spirit that once made ABT the shining example of a crusading and daring artistic enterprise, and which fostered the creative careers of De Mille, Loring, Tudor, Robbins, Michael Kidd, and so many others, including, yes, Tetley?

Admittedly, the price of risk has escalated, like everything else. But without it, ABT courts stagnation, and worse the petrifaction of its own heritage. It is this tendency one must hope will be reversed with the coming change of regime. CAPTION: Picture, no caption