Fourteen newborn female, and ten male pigs were injected subcutaneously with 100 mg of testosterone propionate within 24 h of birth, and ten male and nine females were used as controls and injected with vehicle alone (Sesame oil). All animals were slaughtered at 175 days of age. Reduction of testicular weight, tubuli size and testosterone concentration, together with alteration of the intertubular area was found in adult treated males. In adult treated females, the histological study of ovaries showed the parenchyma fully occupied by multiple Graaf follicles, but no corpus luteum was found. In addition, treated females never had serum progesterone levels above 2 ng/ml, whilst control females, in some cases had values close to 2 ng/ml, and in some others near to 10 ng/ml. Carcass weight and fatness were not markedly different between groups. The observations presented in this report provide further support for a neonatal component of testosterone-dependent masculinization process in pigs.