Previous research on sexual behavior of the great apes suggests that female chimpanzees readily conceive in traditional laboratory pair tests, i.e., free-access tests (FATS), but female gorillas and orang-utans do not. Although several different mechanisms could account for this result, the one tested is derived from R.V. Short's hypothesis that “form reflects function,” with respect to the ratio of testis weight to body weight in the great apes. The implication of this hypothesis is that the more frequent maleinitiated mating that occurs prior to ovulation in the FATs, in comparison with the wild, reduces the fertility of male orang-utans and gorillas as a function of their relatively small testes. The hypothesis was tested by conducting restricted-access tests (RATS) with female choice, tests in which the female determines whether and when copulation may occur. In the RATs, the cycle rate of copulation was reduced in both the gorillas and orang-utans to rates typical of their wild counterparts. Three females of each species, moreover, did conceive in the RATs, but this result was not statistically significant. Several conditions were implicated in the failure of additional females of both species to conceive during the RATs, including an absence of copulation, copulation without ejaculation, and copulation with ejaculation temporally dissociated from the presumptive time of ovulation. Little support was obtained for the hypothesis that failure to conceive during the FATs was due to the males' relatively small testes and frequent copulation, but such a possibility cannot be ruled out. Relatively low levels of pregnanediol glucuronide during the luteal phase of the menstrual cycle were implicated in the failure to conceive by female gorillas but not female orang-utans. The data suggest that female regulation of mating in the RATs results in patterns and rates of copulation that are more characteristic of these species than those obtained in the FATs. Restricted-access testing thus may prove useful in facilitating the breeding of gorillas and orang-utans in other captive settings.