Laboratory studies of sexual behavior in gorillas and orang-utans produced few pregnancies despite the occurrence of mating, including mating during the midcycle, which is the presumptively ovulatory phase of the cycle. Comparison of laboratory and field data suggests that for both these species, as well as for the chimpanzee, mating occurs more frenquently throughout the cycle in the laboratory than in the field. This increased mating occurs as a result of relatively increased male sexual initiative (chimpanzee and orang-utan) or male intimidation of the female (gorilla), made possible by (1) male dominance over females, and (2) testing in a single cage which prevents the female from avoiding or escaping the male. Some limited data on gorillas suggest that the probability of conceptions is increased when the frequency of copulation in the cycle is decreased. Preliminary data from an ongoing laboratory study of orang-utans suggest a similar conclusion for this species. Several pregnancies were produced using a limited access test condition in which the female, rather than the male, determined whether and when copulation could occur. Under these conditions, mating occurred less freqeuntly and was more restricted to the midcycle phase than during the earlier testing in a single cage which compromised female options regarding mating. The data suggest that the failure of some gorillas and orang-utans to produce offspring, despite mating during midcycle, is due to a reduced capacity of the males of these species to impregnate females as a result of relatively increased copulation prior to the time that the female ovulates. The male gorilla and orang-utan differ from the male chimpanzee in this respect, and this difference is related to differences in testis/body weight ratios and species-typical patterns of mating.