1. Specific binding of [3H]-prazosin in prostatic and aortic membranes of humans was saturable and of high affinity (prostate: apparent dissociation constant, Kd = 0.35 +/- 0.03 nmol/L; aorta: Kd = 0.26 +/- 0.03 nmol/L). The density of [3H]-prazosin binding sites (Bmax) for prostate and aorta was 546 +/- 31 and 61.6 +/- 1.6 fmol/mg protein, respectively. 2. Prazosin, YM617, naftopidil and urapidil competed with [3H]-prazosin for the binding sites in a dose-dependent manner in the prostate and aorta of humans. The binding affinities of these antagonists in both tissues were compared, based on the inhibition constant, Ki. Both prazosin and urapidil showed similar affinity to [3H]-prazosin binding sites in human tissue, whereas YM617 and naftopidil showed approximately a 12 and two times higher affinity, respectively, to alpha 1-adrenoceptor sites of prostate than aorta. 3. The chloroethylclonidine treatment reduced partially the Bmax values for specific [3H]-prazosin binding in the prostate and aorta of humans with little effect on the Kd values. 4. These data suggest that YM617 is a relatively selective antagonist of human prostatic alpha 1-adrenoceptors.