Urinary steroid hormone levels were measured in wild male mountain gorillas (Gorilla gorilla beringei) to determine how levels of testosterone and cortisol corresponded with age and social rank. Urine samples were collected noninvasively from 18 males, ranging in age from 3-26 years, in three groups of wild mountain gorillas at the Karisoke Research Center, Rwanda, Africa, and samples were analyzed using radioimmunoassay procedures. Males were classified as being immature (< 7 years), maturing (10-13 years), or adult (+13 years of age). Immature males had significantly lower levels of testosterone and higher levels of cortisol than both maturing and adult males. No differences in testosterone or cortisol levels were found between maturing and adult males. Dominant males exhibited a trend toward significantly higher levels of testosterone than subordinate males, but no difference was found between cortisol levels of dominant and subordinate males. These results suggest that the increase in testosterone associated with puberty occurs prior to any outward sign of development of secondary sexual characteristics. Within-group male-male competition may affect testosterone levels, but the lack of difference in cortisol levels between dominant and subordinate males suggests that subordinate males are not socially stressed, at least as measured by cortisol.