To compare the behavior of pancreatic enzymes in the circulation, serum levels of pancreatic isoamylase activity, lipase activity, immunoreactive trypsin (IRT), and immunoreactive elastase 1 (IRE) were measured in the same serum samples taken serially from 29 subjects undergoing endoscopic retrograde pancreatography (ERP). A striking correlation between the maximal increments of serum pancreatic enzyme levels after ERP and the degrees of opacification of pancreatic duct system was observed, except in 6 subjects with chronic pancreatitis. In 13 out of 19 subjects whose main pancreatic duct (MPD) and branches were opacified, serum pancreatic enzyme levels reached a peak within 2 hours after ERP and decreased thereafter. The mean maximal rise of serum levels of lipase, IRT, pancreatic isoamylase, and IRE in the 13 subjects was 32, 21, 10, and 4 times the basal value, respectively. A delay of peaking in serum levels of pancreatic isoamylase and IRE as compared with those of lipase and IRT was observed in the 13 subjects. The mean disappearance half-time of serum lipase, IRT, pancreatic isoamylase, and IRE in the 13 subjects was 2.8, 4.6, 8.0 and 16.0 hours, respectively.