Administration of 150 mg×2 of testosterone cyclopentyl propionate to 10 normal males produced significant decreases in thyroxine- binding globulin, cortisol-binding globulin, trypsin inhibitor, transferrin and fibrinogen. It had no effect on total serum proteins, albumin, immunoglobulins, ceruloplasmin, haptoglobin and plasminogen. The decrease in CBG was accompanied by a decrease in total serum and urinary 17-OHCS, but there was no change in serum non-protein-bound cortisol. There were no changes in urinary amino acids, but there was a significant increase in plasma proline and glutamic acid and a decrease in plasma tyrosine. The observed results, when compared with the effects of administered estrogens, were only partly opposite.