Changes in vascular volume of dog jejunum caused by norepinephrine, isoproterenol, or acetylcholine at constant=flow perfusion, were compared to changes in volume caused by changes in blood flow or venous pressure. Vascular volume was measured by indicator dilution mean transit time, using a step input of indocyanine green (125 microgram/min). Venous pressure was held at 10 mmHg; control arterial pressure was about 110 mmHg. At a control flow of 521 ml/min.kg of tissue, the vascular volume was 104 +/- 14 (SD) ml/kg of tissue. Reducing flow by 75 percent caused the volume to decrease by 29 percent (--31 ml/kg); maximal norepinephrine infusion at constant flow and venous pressure decreased the vascular volume by 24 percent, and a 10-mmHg reduction in venous outflow pressure caused a 25 percent (--27 ml/kg) reduction. On the other hand, isoproterenol (100 microgram/liter) at constant flow caused a 245 percent increase in conductance and only a 12 percent increase in vascular volume. Thus, active venoconstriction, changes in venous pressure, or changes in flow independently may cause changes in vascular volume of the intestine. Active smooth muscle changes in the venous capacitance vessels are not necessarily correlated with changes in the arterial resistance vessels.