We have studied testicular function in the biotin-deficient rat biochemically and morphologically. Serum testosterone and luteinizing hormone (LH) levels were decreased significantly in the deficient rats. Administration of biotin or gonadotropins to the deficient rats reversed this decrease in serum testosterone. There was no difference in the serum cholesterol level between the control and biotin-deficient rats. A significant degree of sloughing of seminiferous tubule germinal epithelium was noticed in the biotin-deficient rat testes. Biotin treatment of biotin-deficient rats reversed this condition whereas testosterone treatment was without any effect. The development and maintenance of morphological and functional integrity of the seminiferous tubules appears to require a biotin-mediated step in addition to testosterone.