We studied the effects of amrinone on contractility and aftercontractions of ouabain-superfused ventricular muscle from neonatal and 3-month-old dogs. In 3-month ventricular muscle, amrinone, 5.3 X 10(-4) M, alone, increased active tension by 150%. When amrinone was superfused over ouabain-treated ventricular muscle, the increment in contraction was greatest for those muscles in which ouabain had induced a small positive inotropic effect and diminished as the positive inotropic effect of ouabain increased. Amrinone alone did not induce aftercontractions but increased the amplitude of those induced by ouabain by 92 +/- 6%. In neonatal ventricular muscle, amrinone alone was negatively inotropic and it decreased the increment in active tension induced by ouabain.