Although cocaine is a common drug of abuse, its effects on the testicular structure have not been studied in depth. We report here the testicular lesions produced by chronic cocaine administration to rats. Twenty-eight male Wistar rats weighing 225-250 g were used throughout. Fourteen of them (controls) were injected i.p. with saline solution and the remaining 14 received 30 mg kg-1 day-1 i.p. of aqueous cocaine hydrochloride solution. Animals from both groups were sacrificed at variable times: 7, 15, 30, 45, 60, 75 and 90 days of treatment. Histopathological examination of the testes showed initial alterations at day 15: capillary dilatation; interstitial oedema with lipoid drops due to incipient necrosis of interstitial cells; and necrosis of the cells of the seminiferous tubules. Lesions progressed until 90 days of treatment, showing atrophic and necrotic cells and terminal tubule fibrosis. At this time, the testes were found to be diminished in size. The seminiferous tubules were shrunken, of small diameter and contained necrotic spermatogenic cells and fibrosis. These lesions may produce testosterone diminution and thus sterility due to the disappearance of spermatogenesis.