This study was initiated to determine the causes of lowered reproductive efficiency in mice selected for increased post-weaning gain. Four experimental groups were established: (1) Group C, the two control lines; (2) Group S, the four selected lines; (3) Group CS, the F₁ progeny from the 4 control line X selected line crosses; (4) Group SS, the F₁ progeny from 6 selected line X selected line crosses. Observations were as follows: (1) Estrous cycles were charted by vaginal smears and plugging rate, litter rate, and litter sizes recorded; (2) Ovulation rate was obtained by counting CL's at 3½ and 7 days after mating; (3) Fertilization rate was obtained at 3½ days after mating; (4) Pre- and early post-implantation losses were obtained at 7 days after mating; (5) Late post-implantation losses / were obtained 17 days after mating. Results were analyzed by analysis of variance. No significant differences between experimental groups were found for ovulation rate, fertilization rate, or embryonic mortality. Highly significant (P < .01) group differences were found for the number of days from introducing the male until a CP was observed. Group C plugged in an average of 3.39 days, whereas group S averaged 5.50 days. The number of days spent in proestrus, estrus, and metestrus were each significant (P < .01) for group differences, whereas diestrus was not significant. Plugging rate and litter rate, differed significantly (P < .01) as did litter size (P < .05) among the experimental groups. / Master of Science