The onset of sexual cycle postpartum was described in the collared peccary (Tayassu tajacu). Serum progesterone and 17β-estradiol profiles, vaginal smears and external genitalia were analyzed in 20 animals housed with their piglets during the first postpartum month. The appearance of external genitalia showed no variation in any of the females: a shallow, reddish vulva, and vaginal mucus were constant features throughout the study. Based on hormonal profiles and vaginal smear cell patterns, 16 (80%) of the 20 peccaries showed signs of estrus and were considered cycling. The remaining four females (20%) did no show signs of estrus confirmed by low levels of progesterone (0.9 ± 0.4 ng/mL) during the first postpartum month. In the cycling peccaries, a serum 17β-estradiol peak (53.4 ± 8.1 pg/mL) was observed on Day 7 ± 1 postpartum, along with a linear increase in progesterone concentration from 3 (4.3 ± 2.6 ng/mL) to 11 (30.8 ± 4.9 ng/mL) days after this estradiol peak. Proportions of the different cells of the vaginal epithelium also changed in these females: superficial plus intermediate cells amounted to 76% of the cell total between Days 6 and 9 postpartum, corresponding to the estradiol peak. Nine (56%) of the 16 cycling females mated, indicated by the presence of sperm cells in their vaginal smears, and 6 (67%) became pregnant, reaching term. Non-pregnant cycling females (n = 10) showed a steady decrease in serum progesterone concentration from 11 to 23 days after the estradiol peak, when basal levels were attained and a new estradiol peak registered, indicating the resumption of cyclicity in these females. The time interval between the two estradiol peaks was 23.5 ± 2.1 days in these females. In pregnant females, progesterone concentrations continued to rise to levels of 60 ng/mL (n = 6) 23 days after mating. These findings indicate that the lactating collared peccary female can become cycling and fertile during the early postpartum period, and that a predominance of superficial plus intermediate vaginal cells can be taken as the first sign of estrus. © 2006 Elsevier Inc. All rights reserved.