[3H]‐prazosin binds specifically to a single class of α‐adrenoceptors in rat cardiac membranes (KD25°C = 0.2 nM). That these receptors are of the α1‐type was indicated by competition studies, i.e. α1‐antagonists such as prazosin and (2‐(2, 6‐dimethoxyphenoxyethyl) aminomethyl‐1, 4‐benzodioxane (WB 4101) were more potent than the α2‐antagonists, yohimbine and piperoxan in inhibiting [3H]‐prazosin binding. A comparative study of [3H]‐prazosin binding and [3H]‐dihydroergocryptine binding to cardiac membranes showed that both [3H]‐prazosin and [3H]‐dihydroergocryptine (at low concentrations) bind to α1‐adrenoceptors, while [3H]‐dihydroergocryptine (at higher concentrations) also binds to another class of sites.