Effects of vasodilating drugs on microcirculation of the rat cremaster muscle were investigated microscopically. Topical application and intravenous injection of papaverine produced dilatation of arterioles. Cyclandelate applied topically dilated the arterioles to a considerable extent. Topical application of bamethan induced arteriolar dilatation while bamethan given intravenously constricted the arterioles. Kallikrein applied topically induced a slight dilatation of arterioles, and intravenous administration of kallikrein produced an appreciable vasodilatation. Topical administration of bradykinin produced a vasodilatation of arterioles of the rat cremaster muscle. These results indicate that direct action of a drug on the microcirculation can be properly evaluated by the microscopic method in the rat cremater muscle, if the drug is applied topically, in the vicinity of small vessels under study.