The inotropic response to amrinone and milrinone in isolated cat papillary muscle is characterized by a dose-dependent increase in contractility, with milrinone about five times as potent as amrinone, no effect on load dependence of relaxation, no change in timing and duration of the contraction-relaxation cycle, and marked temperature dependence. This response necessitates, at least in part, the presence of a well-functioning sarcoplasmic reticulum (SR). Amrinone and milrinone are less active when the SR is poorly developed, as in frog myocardium, mammalian atrial myocardium, Purkinje fibers, and ventricular muscle from fetal and newborn animals; when the SR has been destroyed, as in single mammalian cardiac cells; and when the SR, for reasons still under investigation, has become inactive, as in isolated human ventricular myocardium. Amrinone and milrinone are also less active or may depress contractility under conditions in which the SR is known to function near maximal calcium saturation (as in rat ventricular myocardium) or to be overloaded with calcium (as during reoxygenation). This depressant action suggests concomitant desensitization of the contractile proteins to calcium.