The sperm-rich fraction of stallion semen was collected in an AV and, after dilution in an extender, was cooled to 2--5 degrees C before placing in aluminium tubes for freezing in liquid nitrogen for several hours or months. The spermatozoa in about 200 ejaculates from 36 stallions were examined to compare their survival time, motility and velocity before and after thawing. According to the various indices used, 20% of stallions produced spermatozoa which were unaffected, 60% partly but not seriously affected and the remainder completely inactivated. The velocity of spermatozoa decreased from 51.4 micrometers/sec in the fresh semen to 36.8 micrometers/sec in the thawed semen. The fertilizing capacity of the spermatozoa of frozen--thawed semen of 5 stallions was examined in 14 mares. In all, 65 inseminations were made and the blastocysts were recovered non-surgically from the uterus 7--9 days after ovulation. A 20% drop in blastocyst recovery occurred as the result of freezing and thawing, when the same mares were used for insemination of raw and frozen--thawed semen. The capacity to freeze sucessfully proved to be a specific characteristic of certain stallions. Degenerate blastocysts were not recovered but those resulting from artificial insemination of frozen semen were much smaller in diameter than those following insemination of raw semen.