Abstract: A yellowish-brown, amorphous substance isolated from Lo-pu-ma (Apocynum lancifolium Rus.) has been found to show a cardiac action similar to that of cymarin, but its potency and cumulative effect are quite different from those of the latter. The cat unit of the substance is 3.19 mg/kg, being twenty times greater than that of cymarin (0.16 mg/kg). It has a definite cumulative effect, while cymarin practically has none. It is therefore presumed that the substance under study may be a glucoside quite different from cymarin, or more probably it may be a mixture of glucosides which we have not yet separated.