One hundred and seven female rabbits were divided into three groups and irradiated with 400 r from a radiocobalt source. The first group (71 animals) was irradiated before or 4–12 hours after mating and an ovulating injection of H.C.G. The second group (27 animals) were irradiated at about the same time after injection but inseminated after irradiation. The third group (9 animals) were inseminated with spermatozoa which had been irradiated in other females.

The ova were recovered at various times for the determination of fertilization, cleavage, and blastocyst formation. Groups of animals were autopsied 14 or 29 days later for the determination of fetal development. The fertilization rate was not disturbed whether sperm, oocytes, or both were irradiated. The disturbance of nuclear material, the aberration of chromosomes, the retardation of cleavage, the fragmentation of cytoplasm, and the failure of blastocyst formation and embryonic development were observed especially whenever oocytes were irradiated. Irradiation of oocytes before mating and injection appeared to be less harmful but no differential effects were observed when irradiation was at 4, 6, 8, 10 or 12 hours after ovulating injection or when oocytes alone or both the oocytes and sperm were irradiated, probably due to the dosage being too high. Irradiation of sperm in the female tract was less harmful, normal blastocysts and apparently normal fetuses were obtained when nonirradiated ova were fertilized by such spermatozoa.