Time-effect relationship and sensitivity of pancreatic exocrine secretion and pancreatic blood flow were established, to test the supposition that blood flow to the pancreas is controlled by the secretory process. Pancreatic exocrine secretion rate was stimulated by incremental iv doses of secretin (Karolinska, 0.001--4 U/kg) in pentobarbital anesthetized dogs. Arterial pancreatic blood flow was measured by electromagnetic flowmeters; secretion rate was determined by measurement of frequency of consecutive secretory drops. Exocrine secretion rate and blood flow increased markedly. Secretion responded at a dose level 3-7 times lower than blood flow did. So at low secretory performance, the pancreas seemed to be well provided with basal blood supply. At moderate and high secretory levels blood flow did increase, but the flow increase was seen 16-22 sec before the secretion increase. So the additional support of blood flow in the secretory process occurred during the cellular preparation and during the start of the secretion increase. This suggests that secretion increase and blood flow increase are mediated by independent processes or receptors, but these processes seem to cooperate in processing the pancreatic juice.