Animal studies have shown that pancreatic enzyme secretion is independent of enzyme synthesis. To investigate this relationship in humans, we have coinfused 14C-labeled leucine tracer with cholecystokinin octapeptide in nine healthy adults for 4 h and measured the rate of appearance of secreted and newly labeled enzymes in the duodenum. Enzyme secretion was well maintained throughout, but newly labeled enzymes only appeared in juice between 75 and 101 min (median time, 86 min), indicating that initial secretion was dependent on the release of zymogen stores and that the median production time for new enzymes was 86 min. Between 85 and 225 min there was a curvilinear increase in the enrichment of secreted enzymes with newly synthesized enzymes, suggesting a median turnover rate of zymogen stores of 29%/h (range 12-47%/h). In conclusion, our results suggest that in healthy humans, postprandial pancreatic enzyme secretion is maintained by the export of a large stored pool and is not rate limited by enzyme synthesis, since it takes approximately 86 min for newly synthesized enzymes to take part in the digestive process.