The early neonatal development of boars is characterized by significant testicular production of androgens and estrogens, including an anabolic steroid hormone, 19-nortestosterone. The present study was conducted to determine the expression and presence of steroidogenic and steroid hormone metabolism-related enzymes in the testes of neonatal and 4-month-old prepubertal pigs. Quantitative analyses with real-time polymerase chain reaction and Western blotting were utilized to reveal mRNA and protein expression, respectively. The localization of the molecules in the testes was determined by immunohistochemistry. mRNA expressions of the molecules tested were mostly significantly increased between 1 and 3 weeks of age and decreased at 4 months of age, compared with those at 0 weeks of age. The protein levels of cytochrome P450 aromatase and carbonyl reductase 1 were significantly increased between 1 and 3 weeks of age and decreased at 4 months of age. However, protein expression patterns of other molecules differed from those of mRNA expression, which implied the existence of posttranscriptional gene regulation. Immunohistochemical analysis revealed that all of the molecules were present in Leydig cells of the pig testis, regardless of age, except cytochrome P450 side chain cleavage in germ cells and 17beta-hydroxysteroid dehydrogenase 4 on the blood-testis barrier at 4 months of age. Aldose reductase and 3beta-hydroxysteroid dehydrogenase were localized in both Leydig and Sertoli cells. We postulate that marked rises in the expression of steroidogenic enzymes in the pig testis during early neonatal development could be associated with peak production of 19-nortestosterone, thus eventually leading to the early growth of male pigs.