Testicular steroidogenesis and spermiatogenesis during developmental and seasonal changes were investigated in male sika deer (Cervus nippon), a short-day seasonal breeder, to clarify the physiological mechanisms for reproductive function. The immunohistochemical localization of steroidogenic enzymes (P450scc, P45007, 3 beta HSD and P450arom), spermatogenesis and cell proliferation were analyzed in the testes of fetal (164 to 218 days of fetal age), fawn (0 years old), yearling (1 year old) and adult (more than 2 years old) male sika deer. Three kinds of steroidogenic enzymes, P450scc, P450c17 and 3 beta HSD, essential for the synthesis of testosterone were located only in the Leydig cells of the testes from the fetal period, and these localizations did not change during developmental or seasonal stages. Immunoreactivity for P450arom, a key enzyme converting testosterone to estradiol, was also localized only in the Leydig cells of testes but was also further limited to the testes of yearlings and adults. Seminiferous tubules had already formed in the fetal testes examined in the present study. Spermatogenesis started in yearlings and was more active in the breeding season. In the adult sika deer testes, the Leydig cells, which displayed immunoreactivities for steroidogenic enzymes, changed to have more cytoplasm in the breeding season than in the non-breeding season. Cell proliferation of Leydig cells was hardly observed in adult testes during seasonal changes. The present results suggested that sika deer testes start to synthesize testosterone from the fetal period, that seasonal changes in testosterone and estradiol syntheses are dependent on the quantitative variation of steroidogenic enzymes synchronized with the size of Leydig cells and that estradiol synthesized in yearling and adult testes makes a contribution to the initiation and recrudescence of spermatogenesis and spermiogenesis in the sika deer.