Three groups of adult Wistar rats were injected every second day with: 5 or 10 or 20 micrograms of tamoxifen (Tx) to the right testis. Left testis was injected with the same volume of solvent. Control group received the solvent for Tx. Animals were killed after 15 days. Blood samples were taken from heart for testosterone determinations. Both testes were taken for histological examination and quantitative analysis of seminiferous epithelium was performed. Testosterone was analysed with radioimmunoassay method. It was demonstrated that intratesticular supply of Tx caused significant rise in the number of type A spermatogonia and significant increase of testosterone concentration in the blood. It is concluded that Tx influences spermatogenesis in adult rats through stimulation of the earliest steps of gamete formation. Testosterone may or may not mediate this effect.