To test the hypothesis that endocrine declines in males are incidental to disease, 24 gonadally intact old (22-24 months) rats were selected on the basis of good general health and assigned to one of three groups. One group of aged males was left untreated for comparison with an untreated control group of young adult males. Results from multiple measures of sociosexual behavior and reproductive physiology indicated that endocrine declines in males are not simply a by-product of increased disease incidence with aging. The untreated old animals showed clear decrements on all 13 measures of hypothalamic-pituitary- testicular (HPT) activity. The other two groups of old males were used to compare responsiveness of the aging HPT axis in healthy males to supplements with a typical exogenous (ExT) androgen regimen (300 micrograms testosterone/kg body wt/SC/daily/6 weeks) or to social stimulation (brief daily exposure to an inaccessible estrous female) for additional episodes of endogenous (EnT) hormone. Neither treatment restored our disease-free old male rats to levels approximating those of untreated young adults. Nonetheless, both treatments activated the aging HPT axis. EnT males showed increases in sociosexual behaviors, growth of androgen-sensitive bulbospongiosus muscle, and elevation of epididymal sperm reserves. ExT males, on the other hand, experienced a more foreboding hypertrophy of the ventral prostate gland. Our conclusion is that endocrine aging in males is ubiquitous and inevitable. Still, aged androgen-sensitive systems of healthy old rats retain notable capacity, particularly, for endogenous activation. Evidence points to functional responses by healthy aged males to the presence of sexually receptive females that, although not quantitatively the same, are qualitatively similar to the responses of young adult males.