We present 12 years of perineal swelling data for a semifree-ranging colony of mandrills (Mandrillus sphinx), and evaluate the influence of rank, parity, and seasonality on reproductive parameters. Female sexual swellings showed a seasonal pattern, with August the median month of ovulation. Overlapping periovulatory periods did not decrease the likelihood of conception. Females showed their first genital swelling at age 3.6 years (n = 28; range, 3.2–4.6 years), and higher-ranking females experienced their first swelling earlier than low-ranking females. Median postpartum amenorrhea (PPA) duration was 208 days (n = 92; range, 74–538 days). PPA was longer in primiparous females than in multiparous females, but PPA duration was unrelated to female rank. Median follicular phase duration was 24 days for the first cycle after parturition (n = 84; range, 12–40 days), shortening to 17 days in subsequent cycles (n = 55; range, 6–39 days). The follicular phase was longer in nulliparous females than in parous females, but was unrelated to female rank. Median cycle length (from one sexual swelling breakdown to the next) was 38 days (n = 57; range, 18–108 days). Eighty-seven percent of conceptions occurred within two cycles, and half of the nulliparous females conceived during their first swelling cycle. Lower-ranking females were more likely to require more cycles to conceive than higher-ranking females. The cycling phase was significantly longer in nulliparous females than in parous females, and was also significantly longer in lower-ranking females than in higher-ranking females. We discuss the influence of provisioning on female reproductive parameters, the influence of parity and rank on the different phases of the interbirth interval, and the evolution of long and variable follicular phases in mandrills. Am J Phys Anthropol, 2004. © 2004 Wiley-Liss, Inc.