Forty-eight untreated rhesus females (Macaca mulatta) were studied from birth until menarche and for the 12 intermenstrual periods following menarche. The 48 animals were studied in two subsets: one in Oregon (N = 22) and the other in Wisconsin (N = 26). The Oregon animals menstruated 127.3 +/- 6.6 (SEM) weeks after birth, whereas the Wisconsin animals reached menarche 133.0 +/- 3.1 weeks postnatum. These ages did not differ significantly nor was there a significant difference in mean body weight at menarche between the two groups. A linear pattern of ponderal growth was observed from birth to menarche with an acceleration in ponderal growth closely following menarche. The first 12 cycles after menarche gradually decreased in length from 13.0 to 13.6 weeks (range between groups) for the first intermenstrual interval to 4.9 to 6.1 weeks (range between groups) for the 12th cycle. In these cycles the percentage of animals ovulating gradually increased with time (0% in cycle 1, 6.3% in cycle 2, 50% in cycle 8 and 90% in cycle 12). These data showed a significant linear correlation (r = 0.9328, P less than 0.01). Although cycle lengths in adults were significantly greater during June through August than during the rest of the year, no effect of season could be ascertained in the pubescent animals. Serum samples from two animals bled frequently from the onset of menarche through the first ovulatory cycle contained preovulatory amounts of estradiol-17 beta in anovulatory cycles. This suggests that follicle growth in occurring but ovulation does not occur during the early cycles of adolescence. These data support the concept that recovery from adolescent sterility in rhesus monkeys takes approximately 18 months and that recovery occurs in a nearly linear fashion within a given population. We hypothesize that the inability of a young female to ovulate reflects an inability to respond to ovarian estrogen by releasing ovulatory amounts of gonadotropin.