Ling-Ling and Hsing-Hsing, the sexually star-crossed pandas of the National Zoo, finally consummated their too-long Platonic union yesterday, proving perhaps, that one should never give up too soon on a troubled relationship.

In a late-morning encounter on one of those misty March days much favored by French film makers, love and/or biology triumphed under the zoo's greening willows as several astonished zoologists and a TV film crew looked on, and two Masai giraffes rubbernecked from a nearby enclosure.

"After all those years!" said Dr. Devra Kleiman, the zoo's reproductive zoologist, sipping champagne afterward in the administration building. "He was halfway through before I realized we had a chance for success."

"On a scale of 1 to 10, this ranks as a 15," said Dr. Theodore Reed, retiring director of the National Zoo, who has tracked America's only pandas through seven years of fumbled flirtation. "What a wonderful sendoff for me."

So unlikely had the union appeared that zoo officials already had a flagon of mail-order seed en route by jet from London's highly touted male panda, Chia-Chia, lest Hsing-Hsing falter once again in the lists of love.

Hsing-Hsing, however, for once needed no help and did what he had long been urged to do.

Then, after swaggering smartly for a time around his yard where the union took place, he went inside for a nap.

Ling-Ling retired immediately to an elevated timber bed on her side of the moon gate that divides the two enclosures, and slept away most of the afternoon.

Back in the office, Kleiman and Reed, who have waited out false panda pregnancies before, cautioned amid the champagne that Washington is still a long way from a baby panda, despite yesterday's great leap forward. Pandas in China and Mexico have been bred successfully in captivity, but attempts elsewhere in the world have met with failure.

Repeated natural matings would raise the possibility of conception to 80 or 90 percent, but it will be June or July before zoo officials can know for sure. Even then Ling-Ling could have a miscarriage or suffer some other calamity. Few things in life, after all, are certain.

But that, they said, need not dim yesterday's triumph.

"Take the credit, Devra," Reed said, "you've spent long enough taking the blame."

Actually, most of the blame has fallen on Hsing-Hsing, who in years past, "was never able to establish an effective posture," as Reed put it, and may have been victimized as well by a poor self-image. Since female pandas only come in heat once a year for about three days, he was under a lot of pressure each spring and usually appeared overanxious, misdirected or just plain inept.

Ling-Ling, for her part, usually appeared restless and distracted, rolling over at inopportune moments or otherwise breaking the mood. Zoo officials tried patience, assistance, artificial insemination and even jealousy in an effort to solidify the relationship, once shipping Chia-Chia in from London to give Hsing-Hsing some space.

But nothing worked. "Failure," Reed said, "begets failure."

Kleiman, however, remained hopeful. Despite their physical problems, she said, the panda couple have remained "quite compatible" and "obviously like each other very much."

Hsing-Hsing, she said, has always had what it takes; he just didn't know what to do with it, and apparently didn't take instruction well. Yesterday, however, when Ling-Ling wandered through the moon gate over to his place shortly before noon and presented herself for this year's try, she was bleating encouragement and he appeared to take heart anew.

It took about two minutes.

"Their vocalizations altered sharply after that," Kleiman said. "Then they separated and went to sleep it off."

About 4:20 p.m. zoo officials opened the moon gate again and witnessed some half-hearted mating attempts reminiscent of past years. The pandas "didn't really seem interested," according to zoo spokesman Ilene Ackerman, and retired separately to a light supper of bamboo shoots, cottage cheese and carrots.

This morning at 8 a.m. they will be united again. Kleiman is still weighing the possibility of augmenting Hsing-Hsing's natural contribution with something obtained through less traditional means, but the hope in the Panda House is that love, after all, has found a way.