Background and Aims: Studies in humans have shown that pancreatic enzyme secretion is reduced during acute pancreatitis. It is not known, however, whether the reduction is due to impaired synthesis or disruption of the secretory pathway. Methods: The rate of secretion and turnover of trypsin was measured in 12 patients with acute pancreatitis of variable etiology and severity (median Ranson's score 2.5, range 0- 5, 4 with severe necrotizing disease) and 8 healthy volunteers by 4-h primed/continuous intravenous infusions of 1-¹³C L leucine, and collection of pancreatic secretions by duodenal perfusion and sampling. Results: Trypsin secretion was reduced from 476(73) to 153(60) units/h (mean(SE), p=0.005) in acute pancreatitis, with the greatest reductions being observed in patients with necrotizing disease (32(7) units/h, p=0.003). The time for newly labeled trypsin to first appear in digestive juice was not, however, delayed in pancreatitis patients (87.2(11.1) vs. 94.7(4.9) min); on the contrary, there was an early appearance of newly labeled trypsin at 30 minutes in patients with severe necrotizing pancreatitis (p<0.05). Calculated zymogen pool turnover was unchanged, but pool size was decreased (p=0.01). Conclusions: Despite low rates of luminal secretion, trypsin continues to be synthesized in patients with acute pancreatitis. Our findings could be explained by post-Golgi leakage of enzymes from acinar cells, or by loss of synthetic function in some cells with preservation in others.