The decline in sexual behavior in the aging male rat is being recorded by a computer-assisted monitor, and correlated with basal and sexually induced plasma hormones, testosterone, LH and prolactin. All three hormones rise significantly during mating in young rats, and in those few old rats which still exhibit sexual behavior. If there is not mating, there is no change in the hormonal response to an estrous female. At the age of 20 months, approximately half of sexually experienced male rats exhibit sexual behavior, half do not. Basal levels of plasma testosterone are significantly higher in the maters, as compared with the non-maters; LH and prolactin levels are not significantly different. We are currently attempting to maintain or restore sexual behavior in senescent rats with testosterone (infused IV into indwelling venous cannulas) in situ while the rat is within the mating arena with a sexually receptive female. In other studies, we will increase androgen levels at an earlier age by use of Silastic implants; these rats will be observed throughout their life, their sexual behavior monitored and their hormones measured. Brain androgens and their receptors (both nuclear and cytosolic, by exchange methodology) will be measured in order to localize critical hormonal changes during senescence.