] Objective To investigate the effects of midazolam on vascular capacitance during hypovolemic shock in an animal model and the underlying mechanism. Methods Thirty male SD rats weighing 350-450 g were randomly divided into four groups : (1) control group (n = 8);(2) sympathetic nervous system block ( SNSB) group received hexamethonium 10 mg? kg-1(n = 8);(3) SNSB + noradrenaline (NA) group in which NA was infused following hexamethonium until BP returned to baseline (n = 9); (4) hypovolemia group in which 20 ml? kg-1 of blood was withdrawn ( n = 5). The animals were anesthetized with intraperitoneal ketamine. Spontaneous breathing was maintained. Femoral artery and vein were cannulated for MAP and CVP monitoring. Mean circulatory filling pressure (Pmcf) was measured. Midazolam 0.1, 0.3, 0.5 and 1.0mg?kg-1 were given iv successively with an interval of at least 20 min between two injections. MAP, CVP, HR and Pmcf were measured and recorded before and 2 min after each midazolam injection. Results In control, SNSB + NA and hypovolemia groups (group 1, 3, 4) midazolam significantly decreased MAP as compared with that before midazolam ( P