Testis size, seminiferous tubules and plasma testosterone concentrations showed conspicuous annual changes in Sika deer of Hokkaido, Japan. The onset of the spermatogenic process occurred in July or August. Spermatogenic activity had already reached its height in late October, at the beginning of the rutting season, and had begun to decline in late December. Spermatogenesis had stopped in February or March. Plasma testosterone concentrations showed very high levels in late October and early November, but was almost at the basal level in February, March, June and December. The wide individual variation of the plasma levels in October suggest pulsatile secretions of testoster-one.