Changes in both total cardiovascular function and regional hemodynamics associated with large doses of intravenous meperidine were studied in conscious dogs in the absence of other drugs or recent surgery. The animals had previously been surgically prepared by placement of aortic catheters and implantation of chronic indwelling electro-magnetic and Doppler ultrasonic flow probes on the aorta and on mesenteric, renal, and iliac arteries. Meperidine, 2 mg/kg, produced mild renal vascular dilation (10% maximum decreases in resistances and 5% increases in blood flow), but did not alter the resistances and flows in the mesenteric or iliac vascular beds. Meperidine, 6 mg/kg, again resulted in renal dilation (maximum 22% decreases in resistance and 18% increase in blood flow). The iliac vasculature also dilated (maximum 39% decreases in resistance and 40% increases in blood flow). However, the mesenteric vasculature constricted (89% increases in resistance and 40% decreases in blood flow). Following 2 mg/kg of meperidine cardiac output and aortic pressure decreased significantly while total peripheral resistance increased significantly. Heart rate remained unchanged. Following 6 mg/kg of meperidine total peripheral resistance and aortic pressure decreased significantly while heart rate increased. Cardiac output increased during the first part of the observation period then subsequently decreased significantly. It is concluded that in unmedicated dogs meperidine has significant effects on total cardiovascular function and on regional blood flow.