Rat pancreatic juice was collected from female, anesthetized Sprague-Dawley rats (200-250 g) during basal secretion and after 45-min intravenous infusion for secretin or secretin with cholecystokinin (CCK). The volume of secretion was measured as well as activity of lipase and colipase. It was shown that the basal secretion of pancreatic juice gave a colipase/lipase (C/L) ratio of about 0.5. Stimulation with secretin or addition of CCK to secretin gave a C/L ratio between 0.4 and 0.6. This was not significantly different from the ratio found in basal secretion. The secretion of lipase and colipase from isolated pancreatic acini was also found to parallel a C/L ratio equal to 0.6 in the basal release and to a ratio between 0.5 and 0.6 after stimulation with CCK, secretin, or carbachol. The total activity ratio of C/L in the pancreas was equal to 0.97. It is concluded that lipase and colipase are secreted in parallel both in vivo and in vitro. The reason for a lower ratio of C/L in the secreted juice compared to the C/L ratio of pancreatic gland is not known.