Serum samples were collected 1-3 times weekly from two Baird's tapirs (Tapirus bairdii) for 6 months in 1987-1988, and for more than 3 consecutive years beginning in 1989 to characterize hormone patterns during the estrous cycle and pregnancy. Based on serum progesterone concentrations, mean (+/-SEM) duration of the estrous cycle (n = 20) was 30.8 +/- 2.6 days (range, 25-38 days) with a luteal phase length of 18.1 +/- 0.4 days (range, 15-20 days). Mean peak serum progesterone concentrations during the luteal phase were 1.35 +/- 0.16 ng/ml, and nadir concentrations were 0.19 +/- 0.03 ng/ml during the interluteal period. Distinct surges of estradiol preceded luteal phase progesterone increases in most (14/20) cycles. Gestation length was 392 +/- 4 days for three complete pregnancies. Mean serum progesterone concentrations increased throughout gestation and were 1.83 +/- 0.13, 2.73 +/- 0.13, and 4.30 +/- 0.16 ng/ml during early, mid- and late gestation, respectively. Serum estradiol concentrations began to rise during mid-gestation, increasing dramatically during the last week of pregnancy. Patterns of serum estriol and estrone secretion during pregnancy were similar to that observed for estradiol. In contrast to progesterone and estrogens, serum cortisol concentrations were unchanged during pregnancy or parturition. Females resumed cycling 16.2 +/- 2.0 days after parturition (n = 4) and, on two occasions, females became pregnant during the first postpartum estrus. These data suggest that the tapir cycles at approximately monthly intervals and that increases in serum progesterone are indicative of luteal activity. The interluteal period is relatively long, comprising approximately 40% of the estrous cycle. During gestation, progesterone concentrations are increased above luteal phase levels, and there is evidence of increased estrogen production during late gestation. The absence of increased cortisol secretion at the end of gestation suggests that this steroid does not play a major role in initiating parturition in this species. (C) 1994 Wiley-Liss, Inc.