Abstract The effect of cyclosporin A (CsA) on rat testes was assessed in Lewis rats which were given 10 mg/kg of CsA im per day. The rats were divided into 13 subgroups, and 1 subgroup was sacrificed each month. The rat testes were measured and examined for histometrical and morphological changes in comparison to controls. Histometrical analysis included testicular cross-sectional surface area, tubular density, tubular diameter, and the amounts of testicular germinal epithelium, lumen, and interstitial tissue. In the parameters examined, there were no overall differences between CsA-treated animals and controls. CsA does not affect rat testicular tissue.