The left ventricular sensitivity to sympathomimetic amines was assessed in 47 patients with ischemic heart disease and varying degrees of left ventricular dysfunction. Patients were divided into 3 subgroups according to their basal ejection fraction (less than or equal to 35%, between 36 and 54%, and greater than or equal to 55%). After injection of a bolus of isoproterenol (2 micrograms), the isovolumic indexes of inotropic state increased significantly less in patients with an ejection fraction less than or equal to 35% than in other patients, but the heart rate changes and the acceleration in the rate of isovolumic pressure fall were comparable in all subgroups. The dose-response curves to cumulative doses of xamoterol, a beta 1-adrenoceptor partial agonist, confirmed that the magnitude of the inotropic response was reduced during beta 1-stimulation in patients with an ejection fraction less than or equal to 35% when compared with patients with a greater ejection fraction. However, the dose of xamoterol necessary to produce 50% of the maximal inotropic response was not increased in patients with an ejection fraction less than or equal to 35% (range, 1.5-5.2 micrograms/kg; median 2.5 vs. median values of 2.3 and 3.3 micrograms/kg in the other subgroups; NS), and there was no shift to the right of the dose-response curve. It is concluded that in moderate ischemic heart failure, the magnitude of the inotropic response to isoproterenol or xamoterol is reduced. The absence of shift to the right of the dose-response curve to a beta 1-partial agonist suggests that this alteration in myocardial performance is not primarily caused by a decrease in beta-adrenoceptor responsiveness.