Calcium-translocating activity of linoleic acid and its lipoxygenase (linoleate: oxygen oxidoreductase; EC 1.13.11.12) metabolites or autoxidation products was determinedin vitroby estimation of45Ca transport from a bulk aqueous to a bulk organic phase. Fresh commercial linoleic acid, tested immediately after removal from a sealed vial, stimulated calcium translocation only at concentrations greater than 1 mM. In contrast,45Ca translocation by linoleic acid exposed to air was detectable at 10 μM. Oxidation products of linoleic acid obtained either by incubation with lipoxygenase or by autoxidation were much less potent than the calcium ionophore A23187. The products obtained by enzymic oxidation of linoleic acid enhanced contractility in the Langendorff-perfused guinea pig heart up to 45% over control (at 3 × 10−8 M). The inotropic response was transient with rapid onset and not affected by the beta-adrenergic antagonist, propranolol. The autoxidation products of linoleic acid increased cardiac contractility up to 43% at 10−6 M. In contrast, fresh linoleic acid caused only a negative inotropic effect at 10−8to 3 × 10−7 M, progressing to contracture at 10−6 M. These findings suggest that conflicting reports on the cardiostimulant effect of linoleic acid may be due to varying levels of the autoxidation products. Linoleic acid metabolitesin vivomay have a physiological role in myocardial function related to their Ca2+-ionophoric activity.