BALB/c mice were castrated at 2 days of age and control animals were sham-operated. Untreated male and female mice were also included for comparison. One-half of the mice in each group were fed on alternate days with 1.5% N-2-fluorenylacetamide suspended in 1% gelatine by stomach tube beginning at 1 week of age for a total of 14 feedings. The experiment was terminated when the mice reached one year old. Approximately 30% of the male mice, but none of the females, developed liver tumors in groups fed carcinogen. This male predominance in the incidence of hepatocellular carcinomas was completely abolished when the animals were castrated neonatally. No lesions were observed in the liver of mice treated with gelatine suspension alone except one with neoplastic nodule. Although it has been observed that liver tumors are produced more readily in younger than in older mice, the present investigation shows that male hormonal environment during early life is more important than the age on the development of liver tumors initiated by carcinogen.