Whole blood testosterone from male rats was assayed by radioimmunoassay and testicular blood flow rate was measured by a miniature friction flow meter. The secretion rate of testosterone from mature rat testis is 4.2 ± 0.6 and 6.7 ± 1.2 ng/g testis/min (x ± SE) in conscious and halothane-anesthetized rats, respectively. Lower testosterone secretion rates observed in restrained conscious rats as compared to anesthetized rats may be due to stress factors. Prostaglandin E2 infusion depressed testosterone secretion rate. However, there was a parallel depression in blood flow through the testis. (Endocrinology93: 874, 1973)