In female rhesus monkeys (n = 12) housed in a seminatural environment, serum gonadotropin and steroid levels fluctuated annually in a pattern indicative of a seasonally restricted period of ovulatory cycles in the fall and winter and anovulatory cycles in the spring and summer. This seasonal endocrine rhythm occurred independent of pregnancy and lactation, although the presence of a suckling infant delayed the onset of ovulation in the fall by 81 +/- 3.7 days (Dec. 4 vs. Sept 14). Except for serum PRL, levels of gonadotropin and ovarian hormones were similar in lactating and nonlactating females during the spring and summer anovulatory months. Serum levels of LH (less than 10 ng/ml), FSH (less than 4 micrograms/ml), and 17 beta-estradiol (E2; less than 30 pg/ml) were low throughout the summer anovulatory period, exhibiting a significant rise approximately 20 days before first ovulation. Serum progesterone levels were low throughout the 100 days before ovulation (less than 0.5 ng/ml) and did not rise until ovulation occurred. PRL levels remained elevated (greater than 20 ng/ml) in lactating females until 50 days before the first ovulation of the breeding season, but were low throughout the ovulatory, anovulatory, and ensuing ovulatory periods (less than 10 ng/ml). During the breeding period, females exhibited from two to six ovulations. Although the first ovulation of the breeding season occurred within a 40-day period for all females, a subset (n = 6) ceased ovulations significantly earlier than the remaining six females (Jan. 26 vs. March 3). The early cessation of ovulation for these females was associated with significantly lower body weight. After the last ovulation, FSH and E2 fell and remained low, at levels indistinguishable from those of the ensuing spring-summer anovulatory period. In contrast, in females who ceased ovulations later in the breeding season, the period following the luteal phase of the last ovulation was characterized by E2 and gonadotropin levels that were intermediate between those of the anovulatory months and normal follicular phase values. Serum progesterone levels were slightly but significantly elevated following the last ovulation for both groups of females. These data indicate that low basal levels of gonadotropin secretion during the seasonal anovulatory period may result from diminished GnRH secretion or from an alteration in pituitary sensitivity to GnRH stimulation. These data further suggest that the timing of ovulations and associated changes in the neuroendocrine system controlling gonadotropin secretion may be mediated by an environmental variable.