Testicular functions were investigated in 23 grandfathers [60--88 yr old; 67 +/- 7.8 (mean +/- SD)], i.e. men with fertility proven earlier in life. They were recruited from a nonpatient population and led an active life, most of them with a permanent partner. The grandfathers were compared with a group of 20 unrelated healthy fathers, 24--37 years old (29.2 +/- 3.2). Whereas sperm density was higher in the older group, there were no significant differences in ejaculate volume and sperm morphology between the younger and older men. Sperm motility and seminal fructose, however, decreased with age. The fertilizing capacity of sperm as assessed in the heterologous ovum penetration test using zona pellucida-free hamster eggs did not decrease significantly with age. Whereas the basal serum testosterone and estradiol levels were not different between the younger and older men, the response to 2 days of hCG stimulation decreased significantly with age. This decrease was observed in older men whether they had frequent or infrequent sexual activity. Basal serum LH and FSH levels were elevated in the older men. The LH response to GnRH stimulation relative to basal; values was significantly reduce, while FSH responses did not change with age. We conclude that sperm counts and fertilizing capacity of sperm are not negatively influenced by old age, at least not in men with sustained sexual activity. However, the pituitary as well as the testis show signs of decreased endocrine reserve capacity in old age.