The distribution of [3H]-hydrallazine (HP) in the rat was investigated using autoradiography from the whole-body, to the electron microscopic level. Intravenous dosing gave rapid and persistent labelling of blood vessels, particularly arteries, whilst radiolabel from orally administered drug was detectable in the vasculature in modest amounts only at 6 h, the longest interval studied. Light microscopic autoradiographs of blood vessels showed silver grains associated with elastic laminae and the marginal region of smooth muscle cells. Analysis of electron microscopic autoradiographs of arteries from rats dosed intravenously (1 h and 6 h) and orally (6 h) revealed the greatest percentage of radiolabel in each case to be associated with the elastic laminae (34.9 to 40.3%). Significant proportions of total radiolabel (between 12.7 and 34.8%) were ascribed to the smooth muscle cells. It is concluded that radiolabel, possibly in the form of intact HP, is accessible to the vascular smooth muscle cells where the vasodilator action of HP is held to be exerted.