Adenosine triphosphate (ATP) and sodium nitroprusside (SNP) are administered to patients to induce and control hypotension during anesthesia. SNP is authorized for clinical use in USA and UK, and ATP is clinically used in other countries such as Japan. We investigated how these two drugs act on the cardiovascular systems of 20 dogs whose hearts had been denervated by a procedure we had devised. ATP (10 dogs) or SNP (10 dogs) was administered to reduce mean arterial pressure by 30% to 70% of control. Before, during and after induced hypotension, we measured major cardiovascular parameters. Hypotension induced by ATP was accompanied by significant decreases in mean pulmonary arterial pressure (p less than 0.001), central venous pressure (p less than 0.001), left ventricular end-diastolic pressure (p less than 0.001), total peripheral resistance (p less than 0.001), rate pressure product (p less than 0.001), total body oxygen consumption (p less than 0.05), and heart rate (p less than 0.001); all these variables returned normal within 30 min after ATP was stopped. Cardiac output did not change. During hypotension produced by SNP similar decreases were observed in mean pulmonary arterial pressure (p less than 0.01), central venous pressure (p less than 0.001), left ventricular end-diastolic pressure (p less than 0.01), total peripheral resistance (p less than 0.001), rate pressure product (p less than 0.001), and oxygen content difference between arterial and mixed venous blood (p less than 0.05), while heart rate (p less than 0.001) and cardiac output (p less than 0.05) were increased. Recoveries of heart rate and left ventricular end-diastolic pressure were not shown within 60 min after SNP had been stopped. Both ATP and SNP should act on the pacemaker tissue of the heart.