Fifty time-mated pregnant rabbits were used to evaluate the factors influencing fetal survival rate (FSR) after in utero experimentation. Only one of the following variables - number of manipulated fetuses per rabbit, gestational age during experimental procedure, method of hysterotomy, and amount of amniotic fluid loss - was respectively modified in the five consecutive groups. Each group consisted of 10 rabbits and identical procedures were performed in each group. The FSR among groups was compared. Although comparison of FSR between two succeeding groups did not reveal a significant difference, comparison of the FSR of the latter two groups with the former two groups revealed a significant difference. While the most substantial improvement in FSR was achieved by modification of the hysterotomy method, all of the factors, cumulatively if not individually, established a significant difference.