Tochukaso is a Chinese traditional medicine composed of a fruit body of Cordyceps sinensis and its parasitic host larva. Tochukaso (C. sinensis) and the cultured mycelia of five species of Cordyceps and four species of Isaria were each extracted with hot water and examined for the inotropic effect on guinea-pig right atrium in vitro system. The extracts from C. militaris and I. felina showed a negative inotropic effect to approximately the same extent as that from Tochukaso. These three extracts also showed inhibitory action on twitch response of guinea-pig ileum and aggregation of human blood platelet. It is suggested that these activities are ascribed to the combination of adenosine, 5'-adenosine monophosphate and several other nucleic acid-related compounds, all of which have been shown to be present in the extracts.