This study confirms our previous data on the effects of sex hormones on mast cell number (MCN) in the testis of frog Rana esculenta. After 15 days of treatment with oestradiol (E2) MCN strongly increases, while testosterone has no effect. After 30 days only a small increase in MCN is observed. These differences could be due to the non-physiological effect of E2 over a prolonged period. We also confirmed a massive increase in MCN after 15 or 30 days of treatment with cyproterone acetate (CPA). This increase in MCN is also observed after administration of CPA with tamoxifen. Ultrastructural analysis of testis shows empty spaces with degenerating Leydig cells in the interstitial compartment and numerous germinal cells completely degenerated, probably apoptotic, in the adjacent germinal compartment. The same effects were observed in testes after treatment with only CPA. Chronic E2 treatment provokes an increase in MCN on day 2. From day 4 to 12 of the treatment, MCN decreases dramatically and many germinal tubules appear strongly disorganised. In conclusion, the present results confirm that E2 treatment induces changes in MCN and chronic E2 treatment modifies the morphology of the frog testes. In addition, blocking androgen receptors with CPA, alone or in combination with tamoxifen, causes a significant increase in MCN, confirming the involvement of androgens in mast cell proliferation and/or differentiation.