The effect of thyroidectomy on testicular morphology was studied in adult male albino rats. Thyroidectomy caused a decrease in testicular weight and body weight, but no difference on relative testis weight. There was a decrease on seminiferous tubulus diameter, while the spermatid/spermatogonium ratio which is highly correlated with the efficiency of spermatogenesis did not change after thyroidectomy. No other morphological changes were observed in the seminiferous tubules of testis.