In male rats, serum testosterone (T) and progesterone (P) levels fluctuate with daily periodicities that appear to be inversely related. To further investigate this interrelationship between serum T and P levels, we studied the effects of exogenous P on serum androgen levels. At 6–8 h after administration of P, serum T and DHT levels were consistently increased without any alterations in the serum LH and FSH levels. Following disruption of the hypothalamo-pituitary-adrenal axis either by adrenalectomy or by anterior hypothalamic deafferentation, procedures known to abolish serum T and P periodicities, P was again effective in raising serum T concentrations without altering the serum gonadotropin values. These results show that P may directly enhance testicular secretion, and thus support the possibility that the observed adrenal influence on daily testicular T secretion pattern may be hormonally mediated via P secretio