Attenuation by a 5 alpha-reductase inhibitor of the activational effect of testosterone propionate on penile erections in castrated male rats.

Daily administration of an inhibitor of 5 alpha-reductase, 17 beta-testosterone carboxylic acid, significantly attenuated the stimulatory effect of testosterone propionate on penile erections displayed by castrated male rats which were restrained in a supine position. This treatment, however, failed to affect mating behavior in tests with sexually receptive females. The inhibitory effect of 17 beta-testosterone carboxylic acid on erection was reversed by concurrent administration of 5 alpha-dihydrotestosterone propionate, suggesting that the 5 alpha-reduced metabolite(s) of testosterone may normally contribute to the activation of penile erection in the rat.