Superovulated immature rats (24 to 25 days old) were inseminated with epididymal sperm before or after ovulation and their eggs were examined at various times. When inseminated before ovulation, the interval between insemination and sperm penetration was 7 to 9 hours; the sperm penetration began approximately 2 hours after the first appearance of eggs in the oviducts and ceased 4 to 6 hours later. When inseminated after ovulation, the interval between insemination and sperm penetration was 5 to 6 hours; sperm penetration started 5 to 9 hours after ovulation and the proportion of penetrated eggs was not increased 4 hours later. The fertilizing capacity of sperm in the immature female tract lasted for 10 to 14 hours, which is shorter than in the mature animals, and sperm penetration occurred 2 to 10 hours after ovulation. The proportion of eggs fertilized, however, was not higher than 55%. A significant greater number of eggs was penetrated by more than one sperm but the number of polyspermic eggs was not increased when insemination was delayed. Epididymal sperm were incubated for various lengths of time and similarly inseminated into the uterine horns of immature rats. The fertilizing capacity of sperm could be maintained in culture for 10 hours but the proportion of fertilized eggs decreased sharply after incubation for 8 hours. Although rat epididymal sperm in a concentrated form could not be capacitated in vitro, they could be capacitated after incubation by intrauterine insemination.