An encouraging experiment in wildlife management - the return of the native P-horse, once abundant, that had become extinct in the wild more than 40 years ago.

Mongolia is a land of the horse. The warriors of Genghis Khan and his successors in the 13th century conquered most of Asia on the backs of sturdy horses.

Today, nomads still mind their flocks from the saddle, and they never look more at home than in a race across the empty distances of the Gobi Desert, a study in fluid motion and the centaurian harmony of man and horse as one.

But even the Mongols never managed to domesticate the wildest of horses, a species known as Equus ferus przewalskii, or P-horse for short. It is one of just two extant species of horse. All the breeds of the familiar domestic horse, from Shetland pony to Clydesdale, belong to the other species, which submitted to the bit and bridle 6,000 years ago. Przewalski's horse (pronounced zheh-VAHL-skee and named for the 19th-century Russian explorer who first identified it) is about the size of a large pony, with a stocky body in shades of tan to tawny, short brown legs and a dark mane that stands straight up.

They once roamed Central Asia, and particularly Mongolia, where they are called takhi.

Recognition of this horse as a separate species did not come until 1879, with the arrival of Col. Nikolai Przewalski in Mongolia. A Russian of Polish descent, he was on a secret mission to beat the British into Tibet, one more foray (unsuccessful, it turned out) in the "great game" of British-Russian imperial rivalry in the 19th century.

The mission's cover as a scientific expedition rescued Przewalski from total obscurity. He correctly identified animal skins brought to him as those from a strain of wild horse, perhaps the last to avoid extinction. At first, some scientists thought it could be the progenitor horse, a distant ancestor of the domestic horse.

But comparisons of mitochondrial DNA show that the P-horse and the domestic horse diverged from a common ancestor 500,000 years ago.

Some scientists classify the P-horse a subspecies, not truly separate.

The P-horse has 66 chromosomes; the domestic horse 64. Dr. Ryder said more detailed investigations had isolated other genetic markers in the P-horse that were not found in the domestic strain. "But the differences are very small," he said.

Scientists agree that the two animals are the most closely related of the equids, the other living today being African wild asses and donkeys, Asiatic asses and three species of zebra. The close relationship, Dr. Lear said, "is reflected in the P-horse's ability to produce fertile offspring with domestic horses."

This is a matter of both concern and opportunity for the reintroduction program in Mongolia.  Also, the introduced horses arrived already carrying domestic genes.

All the P-horses bred in captivity descend from 12 ancestors caught in Mongolia and taken to zoos and wildlife parks in Europe in the early 20th century. But there is no saying how much mixing of the two species has occurred along the way.

Scientists are of two minds over interbreeding and inbreeding.

Waltraut Zimmermann, a biologist at the zoo in Cologne, Germany, which has supplied the program with P-horses, said: "None is strictly pure. Sometimes you can't see it. Sometimes you see it in their tails. If the wild horse mates with a domestic, the hybrid offspring will have only 65 chromosomes. But future generations will be back at 66 again."

Dr. Ryder said a measure of interbreeding was not only inevitable, but perhaps beneficial. "If our goal is to preserve genetic diversity," he said, "it may be that the wild population must have some degree of gene flow from the domestic horse."