The present study has been designed to determine which stages of seminiferous cycle were modified by in vivo intramuscular administration of one or two doses of 25 mg of testosterone enanthate. Testicular weight was significantly reduced after treatment with two doses of testosterone enanthate (p < .05). Assessment of total lengths of stages per 100 mm of seminiferous tubules scored by transillumination showed that testosterone enanthate increased significantly the length of stages VI-VII, but it reduced total lengths of stages I, XII, and XIII-XIV. A dose-dependent reduction in the absolute and relative frequency of stage I was observed with testosterone enanthate treatment, whereas stages VI-VII were significantly increased. Analysis of relative frequencies of stages showed that stages XIII-XIV and stages I-V were significantly below the control value (1), whereas stages VI-VIII were over 1. In summary, results of the present study showed that transillumination is an adequate technique to assess the effect of drugs on spermatogenesis, and that testosterone enanthate modifies the pattern of the stages of the seminiferous epithelium cycle, before testicular weight is affected.