Correlative morphological and biochemical studies were made on the adrenal of rats treated with daily subcutaneous injections of testosterone propionate in a dose of 50 mg/kg for 30 consecutive days. Histologically, there is a thinning of the zona fasciculata with a decrease of cell numbers, swelling of the cell body and the occurrence of large lipid droplets in the cytoplasm. Electron microscopic examination revealed swelling of mitochondria with reduction of cristae and hypertrophy of smooth-surfaced endoplasmic reticulum with cluster formation or whirl-like arrangement. In in vitro biochemical experiments, it was noted that conversion of deoxycorticosterone to corticosterone or 18-hydroxydeoxy-corticosterone was decreased to 45% of the control level in the testosterone-treated group.