AbstractFor the determination of the fertilizing life of ferret sperm in the female tract, gonadotrophin was intraperitoneally injected to induce ovulation at various times after the deposition of epididymal sperm into the uterine horns. The animals were examined 48 or 60 hours after ovulation injection to assess the proportion of fertilized eggs, although a few of them were allowed to deliver young.Intraperitoneal injection of 50 hypophysectomized R.U. of sheep pituitary extracts or 90 I.U. of Human Chorionic Gonadotrophin invariably induced ovulation in ferrets that were in heat. Ovulation occurred about 24 to 36 hours after the administration of H.C.G. and an average of 14.5 eggs with a range of 8 to 24 eggs per animal was obtained.The average number of sperm in the cauda epididymides and the vasa deferentia of nine males was 219 millions with a range of 24 millions at the end of breeding season to 621 millions at the peak of breeding season.The percentage of total eggs that were fertilized was 73, 40, 24, 15, 3, and 0 when the ovulation injection was given at 0, 36, 72, 84, 96, and 108 hours after sperm deposition respectively. Since ovulation occurred 30 hours after ovulation injection the fertilizing life of ferret sperm in the female tract is therefore no more than 126 hours. The percentage of eggs fertilized was lower following the deposition of epididymal sperm than following mating (73% vs. 98%). There is evidence that the fertility of sperm in the female tract was higher if a large number of sperm was deposited, and that the penetration of sperm into the egg facilitated the denudation of the egg.The majority of the penetrated eggs (54 to 84%) were at the pronuclear stage 48 hours after insemination and ovulation injection (i.e., about 18 hours after ovulation), at 60 hours (30 hours after ovulation) the majority of them were either at pronuclear stage (26–100%) or at the 2‐cell stage (46–57%), and at 70 hours (40 hours after ovulation) most of them were at the 4–6 cell stage. The morphology of fertilization in the ferret and the time sequence after sperm penetration were de