Experiments were designed to characterize the predominant subtype of alpha-1 adrenoceptors in human and canine prostate tissue. The chemical (+/-)-beta-([125I]iodo-4-hydroxyphenyl)-ethyl- aminomethyl-tetralone bound in a specific, saturable manner to a single class of binding sites in membranes that expressed recombinant hamster alpha-1B, bovine alpha-1C and rat alpha-1D receptors expressed in rat-1 fibroblasts and to those from prostate tissue. Competition assays with human and canine prostate membranes revealed only a single class of binding sites. Binding affinity in both human and canine prostate most significantly correlated with binding affinity for the recombinant bovine alpha-1C receptor (r = .98 human, .95 canine). Further analysis with leverage plots demonstrated that binding affinity in human and canine prostate tissue is best predicted by binding affinity to recombinant bovine alpha-1C (P < .01 human and P < .001 canine). These data are consistent with a single class of alpha-1 adrenoceptors in human and canine prostate tissue, which is best represented as the alpha-1C subtype.