BACKGROUND: A small change in venous capacitance significantly alters venous return and thus cardiac output. It is therefore important to know the effects of intravenous anesthetics on venous capacitance, particularly in a hypovolemic state. As ketamine does not suppress sympathetic activity, it has been suggested that ketamine may be the drug of choice for anesthesia during hypovolemia. The purpose of this study was to examine the effects of ketamine or sodium thiopental on venous capacitance and total vascular compliance in dogs. METHODS: Twenty mongrel dogs, weighing 10~15 Kg, were divided into two group (ketamine group: 10, sodium thiopental group: 10) of 10 each. Venous capacitance was assessed before and after drug (ketamine 1 mg/kg or sodium thiopental 5 mg/kg) injection by measuring mean circulatory filling pressure (MCFP) in the normovolemia and hypovolemia. MCFP was measured after arresting the circulation by tightening of superior vena cava and inferior vena cava snares simultaneously. RESULTS: As compared with MCFP in the normovolemia and hypovolemia, MCFP was significantly increased by ketamine in the normovolemia and hypovolemia. As compared with MCFP in the hypovolemia, MCFP was significantly decreased by sodium thiopental in the hypovolemia. The slope of the regression line relating MCFP and blood volume was not significantly altered by ketamine or sodium thiopental, which suggests that ketamine or sodium thiopental did not alter total vascular compliance. CONCLUSIONS: These results suggest that ketamine decreases venous capacitance in the normovolemia and hypovolemia but sodium thiopental increases venous capacitance in the hypovolemia.