Corticotropin-releasing factor (CRF) has previously been shown to selectively dilate the mesenteric vascular bed, without affecting other vascular beds. Pancreatic blood flow and islet blood flow were therefore measured separately with a microsphere technique in the two regions of the rat pancreas perfused by the superior mesenteric artery (SMA) or celiac artery (CA) respectively. Intravenous infusion of CRF (0.25 microgram/kg b.w./min) caused an increase in both whole pancreatic blood flow and islet blood flow in the region of the pancreas perfused by the SMA. The fraction of whole pancreatic blood flow diverted through the islets in this part of the pancreas was, however, unaffected by CRF infusion (approximately 10%). CRF did not change either pancreatic or islet blood flow in the CA-perfused part of the pancreas, and did not affect the release of insulin.