The hemodynamic changes occurring during spontaneous micturition were recorded in conscious cats. Arterial blood pressure was continuously measured by chronically implanted arterial catheter, heart rate (HR) by a cardiotachometer, and cardiac output (CO), superior mesenteric (MF), renal (RF), and external iliac blood flows (IF) by chronically implanted electromagnetic flow probes. Spontaneous micturition was accompanied by little change in mean arterial pressure (-9.7 +/- 0.7%), but by a marked decrease in HR (-49.0 +/- 1.2%) and CO (-28.6 +/- 2.5%), and therefore by a marked decrease in total peripheral conductance (-21.0 +/- 3.5%). Visceral and hindlimb blood flows were markedly reduced during micturition (MF, -34.7 +/- 2.1%; RF, -22.6 +/- 1.5%; and IF, -48.7 +/- 1.5%, respectively) due to a marked reduction in regional conductances in both these areas. The vasomotor changes in the regional circulations were prevented by local sympathectomy. Thus spontaneous micturition is associated with marked changes in cardiac function and systemic circulation. Cardiac output is decreased, but diffuse nervous systemic vasoconstriction compensates for this and provides maintenance of arterial blood pressure level.