In a macroautoradiographic study in pregnant and nonpregnant mice the distribution pattern of 14C-Endralazine (6-Benzoyl-3-hydrazino-5,6,7,8-tetrahydropyrido(4,3-c)-pyridazine- mesilate), a new direct vasodilator drug, was investigated, since earlier studies with minoxidil and hydralazine had shown apparent discrepancies between the relatively short elimination-half-life and the long pharmacological effect of the drugs. After a single oral administration of 5 mg/kg 14C-endralazine the initial high amounts of radioactivity in the gastro-intestinal tract, the kidneys and the liver continuously decreased in the course of the 2nd to the 8th hour after application. 24 h after administration the drug was nearly completely eliminated from these organs except the liver. A remarkably high and persistent affinity of 14C-radioactivity to the blood vessel walls was found even 48 h after application which could explain the long acting pharmacological response to the drug. Only low levels of radioactivity were registrated in the fetus of pregnant mice.