Isradipine (PN 200-110) is a new dihydropyridine calcium-entry blocker with powerful vasodilating properties. Therapeutic concentrations do not affect myocardial contractility. However, in vitro studies have demonstrated at higher concentrations negative chronotropic action with only minor dromotropic influence. For this reason we studied the effect of intravenous isradipine (0.3 microgram/kg/min during 30 min) on sinus node and atrioventricular (AV) nodal function in 25 patients. Nine of these patients had normal sinus node function (group I), nine patients were treated with a beta blocker (group II), and seven patients had a sick sinus syndrome (group III). Mean supine arterial blood pressure decreased in all groups; however, in group I not significantly. Spontaneous sinus cycle length decreased significantly in all groups. In none of the patients effective refractory periods of atrium or ventricle were depressed. QRS duration was not significantly affected in any of the groups. There was only a slight, but significant, prolongation of the QTc of maximal 4% (except in group III). We concluded that isradipine has no depressant effect on sinus and AV nodal function in humans, not even in the presence of beta blockade or impaired sinus node function.