Hypercholesterolemia has been shown to cause an increased vascular reactivity to catecholamines, but the mechanisms leading to greater smooth muscle responsiveness are not clear. Since membrane lipids are associated with receptors, a change in receptor structure and affinity for agonists could have been responsible. This study examined pharmacologic characteristics of cardiac beta adrenoceptors in isolated hearts from rabbits which had been fed a high cholesterol diet for 4, 6, or 8 weeks. Parameters measured were the spontaneous rate changes of the isolated right atrium and changes in refractory period of the isolated, perfused ventricles as these were induced by cumulative doses of the beta receptor agonist isoproterenol. The efficacy of a specific beta-one adrenoceptor blocking drug, metoprolol, in blocking these effects of isoproterenol was also evaluated. The rate and refractory period responses to isoproterenol and the blocking effectiveness of metoprolol were the same in all groups, as were the maximum responses induced by the agonist. It thus appears that hypercholesterolemia does not alter membrane beta adrenoceptor responsiveness in rabbit hearts.