Vasopressin decreases blood flow as well as secretory flow in the pancreas. The question raised was whether the blood flow decrease is the determinant of the decrease in secretion or quite the reverse. In pentobarbital anesthetized dogs, secretory flow was first increased to a steady level by infusion of secretin. At this steady state, O2 consumption and O2 extraction were increased, while blood flow remained at the control level, indicating an increase in the area available for exchange i.e. an increase in capillary density. At increasing doses of vasopressin, secretory flow decreased, arterial flow decreased, and O2 extraction increased, while O2 consumption decreased and venous-arterial CO2 concentration difference was not changed. At the same time CO2 transport decreased, CO2 concentration in the secretion was unchanged and CO2 output in the secretion was decreased. The decrease in blood flow was always seen about 25 s before the decrease in secretory flow, strongly suggesting that the decrease in blood flow induced the decrease in secretory flow. A higher dose of vasopressin was required to decrease the O2 consumption (i.e. this effect was less sensitive) than to increase O2 extraction. The decrease in secretory flow and the decrease in blood flow showed an intermediate sensitivity. So O2 consumption seems to be preserved at a high level by the increase in O2 extraction. It is concluded that the vasopressin-induced decrease in blood flow is the determinant of the decrease in secretory flow. This phenomenon is discussed in terms of the model for metabolic control of tissue oxygenation.