Reproductive organ weights and histology were examined in male and female voles, Microtus pennsylvanicus, which had received daily injections of metyrapone (100 mg/kg) or saline for 21 days from 30 days of age. In females receiving metyrapone, body weight and weights of ovaries and uteri were significantly decreased. In metyrapone-treated males, seminal vesicles were 70% lighter than in saline-injected controls. Histological examination of the reproductive organs confirmed inhibition of reproductive development and maturation in metyrapone-treated voles. The results with metyrapone lend further support to the hypothesis that increased pituitary-adrenal activity may impair reproductive function in rodents.