Studies were performed in four dogs with chronic gastric and pancreatic fistulas following intraduodenal perfusion with 10 mmole hr-1 of sodium oleate for 30 minutes. Radioimmunoassay (RIA) of plasma CCK LI was undertaken by an RIA method using labeled, desulfated CCK 8 I125 and an antiserum raised to CCK 8. The detection limit for the assay was 0.25 to 0.5 fmole and the lowest detectable plasma level was 5 to 10 fmoles ml-1. Since there was equal cross-reactivity to gastrin, a gastrin-specific assay also was employed to evaluate any changes in gastrin levels. After oleate infusion the plasma CCK increment above basal was 50 +/- 11 fmoles ml-1, with return to basal levels after 60 minutes. Administration of atropine significantly (P less than 0.01) inhibited the release of CCK in the first 20 minutes. Thereafter CCK release was not reduced. Plasma gastrin values did not change before and after oleate perfusion. Pancreatic protein output increased from 72 +/- 12 to 420 +/- 55 mg/10 min-1 after oleate administration. However, after atropinization the rise in pancreatic protein output was significantly lower (152 +/- 36 mg/10 min-1) (P less than 0.01). We have shown that, using our RIA method, there is a measurable rise in plasma CCK LI after intraduodenal oleate. After atropinization the CCK response was decreased significantly during the first 30 minutes, but was virtually unchanged during the second 30 minutes, when the fall in pancreatic protein output was most marked. We conclude that the cholinergic mechanism which plays a role in the endogenous stimulation of pancreatic protein secretion by intraduodenal oleate cannot be explained simply be decreased CCK release. This mechanism may be hormonal, distinct from secretin, or neural possibly, via activation of an enteropancreatic reflex.