The pancreatic serum evocation test with secretin has regained importance now that it is possible to determine immunoreactive trypsin and pancreatic isoamylase. After secretin stimulation there was a significant abnormal increase in serum trypsin (p less than 0.01) in 34 patients with proven chronic pancreatitis associated with mild to moderate dysfunction (groups I-II), no rise if there was marked insufficiency (group III). Patients with steatorrhoea and obstruction in the region of the head of the pancreas formed a special group because, contrary to other patients in groups III, they had marked serum enzyme rise after secretin. In 24 control subjects with a normal pancreas there was no significant change in basal pancreatic serum enzyme levels with secretin stimulation. Trypsin and amylase reaction patterns differed during secretin stimulation, with a rise in the amylase occurring at the expense of pancreas isoamylase.