The changes in alpha-adrenoceptors in rat vasa deferentia after injections of reserpine and 6-hydroxydopamine (6-OHDA) were examined in binding studies with [3H]WB4101 and [3H]clonidine. On intraperitoneal injection of reserpine (0.5 mg/kg body weight per day, 2 days), [3H]clonidine binding sites increased by about 2.0 pmol/g wet weight; the binding sites were not detectable in vasa from control rats. [3H]WB4101 binding sites showed no apparent change in amount. After a single injection of 6-OHDA (30 mg/kg body weight), [3H]clonidine binding sites amounted to about 1.1 pmol/g wet weight. The data suggest that both reserpine treatment and incomplete chemical denervation with 6-OHDA cause an increase of [3H]clonidine binding sites in the postsynaptic region of rat vasa deferentia. The possibility was tested by completely denervating rat vasa deferentia by chemical treatment and operation. The treatments increased the binding sites to about 0.9 and 1.0 pmol/g wet weight, respectively over the control level. The results imply that the binding sites indeed also increased in the postsynaptic area.