Macaca fascicularis females were monitored daily through four menstrual cycles then orally administered 0, 50, or 90 micrograms/kg/day methylmercury hydroxide (MeHg). Females were monitored through four additional menstrual cycles and after approximately 124 days of MeHg treatment were time-mated to nontreated males. MeHg treatment did not affect menstrual cycle or menses length. The relationship between dosage of MeHg and reproductive outcome approached but did not reach statistical significance. Reproductive failure (i.e., nonconception, abortion) was, however, related to a significantly higher blood Hg concentration than reproductive success for MeHg-treated females. The offspring of MeHg-treated females tended to be smaller than control infants, but no statistically significant decrease in gestation, birthweight, or crown-rump length was observed. None of the females receiving MeHg exhibited signs of MeHg toxicity during breeding or pregnancy. Daily treatment with 90 micrograms/kg/day MeHg for nearly 1 year, however, produced signs of toxicity in four of seven females. Toxicity was related to increased maternal size, duration of MeHg treatment, and a blood Hg concentration of 2.3 to 2.8 ppm.