The present experiment utilized a cross-classified design to test the interactions and relative importance of male presence or absence, female density and three different photoperiods as factors affecting puberty in female house mice. First vaginal oestrus was used as the criterion for sexual maturity. All three factors significantly affected the timing of first oestrus; male presence or absence accounted for 31 percent of the total variation in age at maturity, 9 percent of the variance was attributable to the female density factor and 6 percent to differences in daylength treatments. There were no significant interactions among the treatment variables. These results conform with a general model relating the processes of sexual and morphological development.