A mating model of the male laboratory rat has been developed in our laboratory. Plasma concentrations of LH, prolactin and testosterone rise (in the order named) in blood collected sequentially from an indwelling jugular cannula in a freely moving male rat while he is mating. These hormonal correlates are aberrant (or missing) in non-mating young mature rats who are (1) natural non-maters (hormones do not rise, or if interested in the female, rise erratically); (2) lesioned in the medial preoptic area of the brain (LH and testosterone rise, but prolactin does not); (3) short-term castrated male rats (testosterone, presumably of adrenal origin rises, but LH and prolactin do not). It is proposed that the senescent non-mater will fall into one of these non-mating categories. A comparison of the behavioral and hormonal characteristics of the aged male rat, when exposed to an estrous female, with young mating and non-mating male rats, will make it possible to designate one of these non-mating groups as a simulated aged group (reproductively speaking) and to examine in depth the relation of these hormonal deviations to sexual inactivity. Preliminary studies indicate that it may be possible to restore sexual behavior in a medial preoptic lesioned animal by infusing hormones into the ablated area. It is proposed to compared aged non-maters with young non-maters, using infusion into both indwelling jugular and intracranial cannulas to restore sexual behavior while the male is in the mating arena. The role of adrenal androgens in the sexuality of both young and old animals will also be evaluated. These studies firstly will deepen our understanding of the hormonal basis of activation and maintenance of sexual behavior; and secondly will make it possible to use a young non-mater as a simulated aged male model for a more detailed analysis of the neuroendocrinological basis of sexual inactivity.