The effects of amrinone on human umbilical artery and human myocardium were studied. Amrinone produced dose related increases in tension, dT/dtmax and dT/dtmin in myocardium from patients who were NYHA grade I (n = 1) and II (n = 4). The responses to amrinone of these tissues were similar to the response seen in normal guinea-pig myocardium (n = 34). The drug had no inotropic effect on tissue from NYHA grade III patients (n = 5). The inotropic response of the tissues to amrinone was inversely related to length of history and severity of heart failure in the patients from whom the tissues were obtained. Amrinone caused dose related relaxation of the human umbilical artery. The vasodilator properties, but not the positive inotropic effects of amrinone were detectable at concentrations of the drug obtained during oral therapy (0.4 to 4.0 micrograms X ml-1). These findings support the view that in patients with congestive cardiac failure amrinone acts by vasodilatation with no clinically important positive inotropic effect.