Treatment with Aldrin, an organochlorine insecticide, for 13 and 26 days caused suppression of 3β-hydroxysteroid dehydrogenase (3β-HSD) and 17β-hydroxysteroid dehydrogenase (17β-HSD) activities, along with accumulation of cholesterol in the testicular tissues of adult rats. The same treatment also resulted in a reduction in the nuclear diameter of Leydig cells (LCND) and diameter of seminiferous tubules. A decrease in the weight of testes, seminal vesicles and ventral prostate was also noted. HCG administration in the long-term (26 days) treatment restored the steroidogenic enzymes activity and the nuclear diameter of the Leydig cells. It also reduced the accumulation of tissue cholesterol towards the vehicle-injected controls. The inhibition of steroidogenesis in the testes possibly reflects a decrease in pituitary gonadotrophin release after the treatment with Aldrin. © 1988.