The mechanical and electrical effects of selected yew alkaloids were studied on two different cardiac preparations: the isolated coronary perfused heart and the isolated papillary muscle of the guinea-pig. In the isolated heart, the Winterstein acid type alkaloids 1, 2 and 3 induced electrical and mechanical effects similar to those reported after yew intoxication (negative inotropic effect, block of atrio-ventricular conduction), but the coronary flow was unchanged. Taxine B (1), the most potent compound of this group, reduced cardiac contractility and the maximum rate of depolarisation of the action potential in the isolated papillary muscle, acting as a class I antiarrhythmic drug. In the isolated heart, the cinnamates 4 and 5, corresponding to the degradation products of 1 and 3, exerted arrhythmogenic effect due to a reduction of coronary flow. No alterations in electrical and contractile activities were in fact recorded after perfusion of the isolated papillary muscle with 4. Taxine A (6) and the taxane alcohol 7, corresponding to the terpenoid core of 3 had no significant cardiac effect. Our results suggest that the poisonous properties of the yew tree are probably due to the combined activity of alkaloids of the Winterstein acid type and their corresponding cinnamtes, which can reduce both the excitability and the coronary flow of the heart.