SUMMARY The effects of post-coital, unilateral ligation of the uterine tube and of bilateral ovariectomy on egg development have been studied in fifty rabbits. Ligation and ovariectomy were performed between 14 and 72 hr, and 11 and 30 hr, respectively, after mating. The animals were killed for examination of the eggs between 3 and 10 days after mating. 284 eggs from thirty-five operated rabbits and 214 fertilized eggs from twelve unoperated rabbits were measured with an eye-piece micrometer as fresh specimens. Until 72 hr after mating the zona pellucida and intra-zonal space were relatively constant at 20 and 125 μ, respectively, while the mucin increased in thickness from 11 μ at 24 hr to 103 μ at 72 hr. The diameter of the intra-zonal space increased to 168 μ at 84 hr and 273 μ at 96 hr, whereas the zona pellucida showed a corresponding decrease to 14 and 5 μ, respectively, but it was still present as a very thin membrane on 6-day blastocysts. In most eggs the mucin began to thin between 72 and 84 hr and apparently disappeared before 120 hr. Ten does in which one uterine tube had been ligated were autopsied 10 days after mating. On the non-ligated, control side, 67·5% of the ovulations (fifty-two eggs) were represented as conceptuses, whereas thirty-six eggs (85·7% of the ovulations) recovered from the ligated tubes were degenerated. When twenty similarly treated does were examined between 3 and 5 days after mating, it was found that egg development continued normally for some 84 hr after mating and then collapse of the early blastocyst occurred. Following ovariectomy in fourteen does, egg development was arrested at approximately 84 hr after mating. In the majority of ovariectomized does, the rate of egg transport to the uterus was normal. Treatment of six ovariectomized does with progesterone prevented degeneration only when the eggs reached the uterus; in three of the six progesterone-treated does some of the eggs were retained in the uterine tubes and degenerated.