Summary. In the oestrous female cat, fertilization can take place when spermatozoa are introduced into the vagina up to 49 hr after an ovulation-inducing injection of human chorionic gonadotrophin. The spermatozoa require residence in the female reproductive tract before they are capable of fertilizing the eggs in vitro. No eggs were fertilized by freshly ejaculated spermatozoa. One of four oestrous females was able to prepare spermatozoa to fertilize eggs in vitro after ½ to 1 hr incubation of spermatozoa in utero. All oestrous females' uteri rendered the spermatozoa capable of fertilizing eggs in vitro after 2- to 24-hr incubation. Sixty-one eggs were cultured in vitro with spermatozoa incubated in utero for 2 to 24 hr, resulting in 62 % of the eggs being fertilized.