Sexual behavior declines in old male rats, and testosterone therapy does not restore the behavior to levels found in young males. If as a result of aging, old males have less capacity to aromatize or reduce testosterone, dihydrotestosterone plus estradiol treatment should be more effective than testosterone treatment in restoring sexual behavior in old castrated males. In a test of this hypothesis, the sexual behavior of old (24 months) castrated Fischer 344 males given injections of testosterone propionate (TP) or dihydrotestosterone propionate (DHTP) plus estradiol benzoate (EB) and that of old sham-operated males given injections of vehicle were observed. The DHTP/EB proved to be less effective overall than the TP in increasing sexual behavior in old castrated males. In a second experiment, young (3 months) and old (30 months) males were tested to verify that the reduced effectiveness of DHTP/EB treatment was age-related. Testosterone propionate and DHTP/EB were equally effective in restoring most measures of sexual behavior in young castrated males. In old castrated males, DHTP/EB treatment was no more or less effective than TP treatment in increasing these same measures. Neither hormone increased the behavior of old males to the level found in young males. Since DHTP/EB treatment is less effective than TP treatment in stimulating sexual behavior in old males, a reduced capacity to aromatize or reduce testosterone is not a likely explanation for decreased responsiveness to testosterone in old male rats.