The pancreatic duct or at least parts of this structure can be demonstrated today by sonography in 75-85% of all persons examined. In 84 persons we have now measured the caliber of the sonographically visualized pancreatic duct in the region of the proximal body of the pancreas with special attention to dependence on age. The diameter of Wirsung's duct ranged from 1 to 3 mm (mean 1.9 mm) and increased significantly from the fifth decade of life onwards. After intravenous injection of the hormone secretin, healthy persons usually show a distinct duct enlargement, which also depends on age. Nine persons aged 19 through 35 (median 28) years showed a dilatation of the main pancreatic duct by about 110% following secretion injection. Nine further probands, 50-74 (median 58) years old, had a dilatation of about 70%. Eighteen patients with confirmed chronic pancreatitis and a pancreatic duct diameter not exceeding 4 mm generally showed no duct enlargement after secretin stimulation. We believe that periductal fibrosis, which is common in chronic pancreatitis, is the most important reason for these results. The use of the sonographic secretin test in the diagnosis of chronic pancreatitis should be considered.