Diadenosine tetraphosphate (AP4A) has a considerable vasodilating effect. Its dose was adjusted to decrease mean arterial pressure (MAP) by 20%, 40%, 50%, 60%, and 80% of the pretreatment value, and the effects on hemodynamics and gas exchange were evaluated. AP4A produced a dose-dependent decrease in MAP within the range of doses which decreased blood pressure by up to 60%. Heart rate remained constant at hypotension levels of 40% or less, but decreased significantly at hypotension levels of 50% or more. Cardiac output increased significantly at hypotension levels of 40% or less. Systemic vascular resistance (SVR) showed a significant decrease at that time. No significant changes were observed in central venous pressure and pulmonary capillary wedge pressure at any level of hypotension. Arterial oxygen tension and calculated pulmonary shunt ratio showed no change. Base excess and pH were decreased significantly with a 60% or 80% fall of blood pressure. In conclusion, AP4A seems to act on the resistance vessels and cause a decrease in SVR.