The relationship between two known actions of prednisolone-3, 20-bisguanylhydrazone (PBGH); Na+, K+-ATPase inhibition and positive inotropic effects, was investigated. In electrically driven left atrial preparations of guinea pig heart, the positive inotropic action of PBGH was not affected by beta-adrenergic or histamine antagonists. Pretreatment of animals with reserpine also failed to influence the positive inotropic action of PBGH. Inotropic concentrations of PBGH inhibited Na+, K+-ATPase and the ATP-dependent binding of (3/)-ouabain to Na+, K+- ATPase preparations in vitro. Additionally, ouabain-sensitive 86Rb uptake, an estimate of sodium pump activity was inhibited when sodium-loaded ventricular slices were obtained from Langendorff preparations at the peak inotropic response to PBGH. Guinea pig heart was highly sensitive to PBGH to the positive inotropic action, the inhibition of Na+, K+ -ATPase and (3H)-ouabain binding, whereas rat, rabbit and dog heart were markedly less sensitive. These findings suggest that the mechanism of the positive inotropic action of PBGH resembles that of ouabain and probably involves Na+, K+ -ATPase inhibition, although the mode of interaction of these steroids with Na+, K+ -ATPase may be different.