The effects of early exposure to aflatoxin on sexual maturation were investigated in young male Japanese quail. Quail in Treatment A were fed a soy-corn starter mash with 10 micrograms aflatoxin/g added from 7 to 21 days of age. Treatment B quail were fed this from 14 to 28 days of age. Animals from each treatment were sampled at intervals from 25 days to 70 days of age. Both behavioral and physiological indicators of sexual maturation were monitored including measurements of sexual behavior, cloacal gland area, testes weight, and peripheral concentrations of testosterone. Statistical estimation was conducted to determine the age at which occurred rapid increase in hormone concentrations and morphological variables indicative of sexual maturation. Control animals showed increased testosterone concentration at 26 days and increased testes weight at 27 days. The cloacal gland area enlarged by 34 days and sexual behavior was first observed at 31 days. Animals in the A treatment had rapid testicular growth at 29 days and testosterone concentrations rose after 27 days. Concentrations were depressed compared to those of controls. Cloacal gland hypertrophy occurred after 37 days; initial sexual activity began at 35 days. The B treatment animals began showing evidence of sexual maturation significantly later with increasing testes weight by 36 days, testosterone concentrations by 37 days, cloacal gland area by 37 days, and initial sexual activity at 35 days. This experiment indicates that exposure to aflatoxin at an early age results in a delay in physiological and behavioral sexual maturation. Animals exposed between 7 and 21 days recovered slightly earlier than those treated between 14 and 28 days.