The effects of hydralazine on aortic rupture, hemodynamics and aortic ultrastructure were studied in turkeys fed B-aminopropionitrile (BAPN). A mortality rate of 24% due to hemopericardium and internal hemorrhage in turkeys fed only BAPN increased to 91% when turkeys were fed both BAPN and hydralazine, despite a significant reduction in blood pressure after both drugs. Death rates among turkeys fed BAPN and hydralazine were lowered by adding either dietary propranolol (53%), which lowered blood pressure and dP/dt max, or reserpine (67%), which reduced blood pressure and increased dP/dt max. Striking ultrastructural alterations of collagenous and elastic fibers of the aortic media, which were additive to the effects of BAPN alone, were induced by BAPN and hydralazine. This study demonstrates that a 6-week feeding of high levels of BAPN and hydralazine, which accumulates in vessel walls, can produce vascular injury and increase mortality from hemorrhage in lathyritic turkeys.