The effects of castration on alpha1-adrenoceptors in rat vas deferens were investigated by determining the actions of selective antagonists against the contractions induced by noradrenaline. The results obtained in vas deferens from control rats suggest participation of alpha1A-adrenoceptors as judged by the pA2 values for prazosin (9.6), benoxathian (9.5), 2(2,6-dimethoxyphenoxyethyl) amino-methyl-1,4-benzodioxone hydrochloride) (WB 4101) (9.6), phentolamine (8.4), 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5] decane-7,9-dionedihydrochloride (BMY 7378) (6.7) and by the insensitivity to chloroethylclonidine (100 microM, 45 min). In vas deferens from castrated rats, WB 4101 and spiperone showed slopes lower than 1.0 in the Schild plots, suggesting participation of multiple receptors. In these organs, noradrenaline contractions were partially inhibited by chloroethylclonidine (100 microM, 45 min), indicating participation of alpha1B-adrenoceptors. After chloroethylclonidine treatment, WB 4101 showed a slope not different from 1.0 in the Schild plot, resulting in a pA2 of 9.4, which indicates an interaction with alpha1A-adrenoceptors. It is suggested that castration modifies the functional alpha1-adrenoceptors subtypes in rat vas deferens.