α₁-Adrenoceptors in the rat prostate were characterized by a binding assay using the newly synthesized radioligand [³H]-YM617 (5-[2-[[2[ethoxyring(n)-³H](o-ethoxyphenoxy)ethyl]amino] propyl]-2-methoxybenzenesulfonamide HCl) and an in vitro assay. Specific [³H]-YM617 binding in the rat prostate was saturable and of high affinity (K_D = 61.5±5.9 pM) with 23.2±6.9 fmol/mg of protein as the maximal number of binding sites (B_max). α-Adrenoceptor agonists and antagonists inhibited the binding of the radioligand with the following order of effectiveness: YM617 > prazosin = bunazosin > WB4101 >5-methyl-urapidil = phenoxybenzamine > phentolamine > S(+)-isomer of YM617 > yohimbine > norepinephrine > phenylephrine>methoxamine. α₁-Adrenoceptors in the rat prostate preferred the R(-)-isomer of YM617 to the S(+)-isomer. Preincubation with chlorethylclonidine (CEC; 10^-5M, 10 min) just slightly changed the B_max value for [³H]-YM617 without changing the K_D value in the prostate; however, CEC reduced the B_max in the aorta. In the isolated tissue, pretreatment with CEC (10^-5M, 10 and 30 min) time-dependently shifted to the right the dose-response curve for phenylephrine and decreased the maximal contraction of aortas induced by phenylephrine, but did not shift or decrease those of prostates. The present results indicate that the α₁-adrenoceptors in the rat prostate are mainly CEC-insensitive (α_1A), whereas those in the aorta are CEC-sensitive (α_1B).