The present study was designed to assess displacemental potencies of terazosin and its isomers for α1High and α1Low adrenoceptor subtypes in rat brain, heart, bovine prostate and canine aorta using a radioligand binding assay method. Although no significant difference in pKi values of each terazosin isomer for α1High in canine aorta and rat brain were observed, the displacemental potency of (S-) terazosin for those in rat heart and bovine prostate was stronger than that of (R+) isomer (p<0.01). On the other hand, only in α1Low subtypes of bovine prostate was stronger displacemental potency of (S-) terazosin than (R+) isomer (p<0.05) observed. Thus, these results imply that there is a different affinity between (S-) terazosin and (R+) isomer on the α1High in bovine prostate and rat heart and α1Low in bovine prostate.