A group of vasotropic drugs was surveyed using a new assay method based on the inhibition of endothelaemia increases after a standard vascular lesion in rats. The new test is considerably more sensitive than all previous ones. It provides a common link between all tested vasotropic drugs and points also to their common mechanism of action. All of them have a protective effect on endotherlium based, in some of them at least, on the preservation of the consistency of endothelial cement. The effect is probably mediated by their influence on calcium availability.