Male and female BALB/c mice were treated orally with 5 mg/kg body weight of methyltestosterone for 10 months. Age-matched control animals received no steroid treatment. The livers of all animals were examined by standard histopathological methods. Hepatocellular dysplasia was found in livers of all treated mice, but not in control animals. The extent and severity of this dysplasia showed a sex difference. Most of the male mice (55/78) developed severe dysplasia and in 23/78 the dysplasia was only moderate; the corresponding numbers for female mice were: mild, 15/71; moderate, 48/71 and severe dysplasia, 8/71. Microscopic hepatocytic nodules were found in 6/78 males and 7/71 females. Hepatocellular carcinoma was seen in one male mouse. These findings indicate that methyltestosterone appears to act as a weak total hepatocarcinogen under the experimental conditions used.