[3H]Prazosin bound with high affinity to the membrane fraction derived from the rabbit ventricular myocardium. Oxymetazoline displaced [3H]prazosin from its binding site, did not elicit a positive inotropic effect but antagonized the positive inotropic effect of phenylephrine mediated by alpha-adrenoceptors in the presence of a beta-antagonist. Naphazoline was more potent in displacing [3H]prazosin and behaved as a weak partial agonist. YM-12617 (5-[2-[[2-(2-ethoxyphenoxy)ethyl]amino]propyl]-2- methoxybenzenesulfonamide HCl), a potent selective alpha 1-antagonist, displaced [3H]prazosin and antagonized the alpha-mediated positive inotropic effect with equal potency. Thus, a good correlation was found between the potency of alpha-antagonists to displace [3H]prazosin and their ability to antagonize the alpha-mediated positive inotropic effect. On the other hand, there was no significant correlation between the Ki and the pD2 value of the alpha-agonists (norepinephrine, epinephrine, phenylephrine and naphazoline), indicating that there is a non-linear relationship between agonist binding to myocardial alpha 1-adrenoceptors and subsequent functional changes. Myocardial alpha 1-adrenoceptors showed some pharmacological characteristics which appear to be different from those in smooth muscle tissues.