Effects of a new selective beta1 partial agonist, ICI 118,587, on cardiac function were assessed in clinical settings. In 7 patients, responses to multistage treadmill exercise were studied before and after an acute intravenous injection of the drug. The heart rate and blood pressure were not altered by ICI 118,587 at rest but increases in both parameters in response to exercise were significantly reduced. Neither oxygen consumption nor plasma norepinephrine level was modified by the drug both at rest and during exercise. The long term effects of ICI 118,587 were assessed in 6 patients with mild to moderate cardiac failure consequent upon ischemic heart disease. After chronic administration of the drug exercise duration was increased. The symptom-limited maximal oxygen consumption increased by 16%, associated with prolongation of the exercise tolerance. In those patients who also had symptoms of angina pectoris, exercise levels which caused angina during the control study were tolerated without symptoms after ICI 118,587. Twelve patients with nocturnal bradycardia resulting from atrial fibrillation of sick sinus syndrome were treated with ICI 118,587. Monitoring of heart rate by 24-hour Holter ECG showed that ICI 118,587 increased minimal heart rate during sleep. Being a beta1-adrenoceptor partial agonist, it has both agonist and antagonist properties. Thus, ICI 118,587 buffers the heart from an excessively low sympathetic tone which may occur during sleep and from an excessively high tone during exercise. It appears to be of benefit in the treatment of mild to moderate cardiac failure consequent upon ischemic heart disease. It also improves oxygen demand-supply imbalance without inducing further myocardial depression or inappropriate bradycardia at rest. ICI 118,587 may therefore be described as a cardiostabilizer.