AbstractEthyl acetate soluble extracts from the leaves of the breadfruit Artocarpus altilis exerted a weak, negative chronotropic effect (p<0.05) and significantly reduced left ventricular pulse pressure (p<0.001) in vivo in the rat. The electrocardiogram maintained a high amplitude QRS complex (ventricular excitation) even when the ventricular pressure pulse was very depressed. On right ventricular myocardial strips, the same extracts produced a significant negative inotropic effect (p<0.0001). This indicates that the in vivo effects might be due in part, to a direct inotropic effect on the myocardium. An in vivo side effect was extensive intravascular haemolysis and consequent haemoglobinuria which could be caused by the vehicle, the extract, or a combined effect of the two. The mechanism of action of the inotropic agent was not cholinergic, and may involve a decoupling of excitation and contraction.