Tissue samples from seven patients with chronic pancreatitis were studied by light and electron microscopy, using samples from the pancreas of seven organ donors as a reference group. Tubular complexes were observed in four of the patients with chronic pancreatitis. Tubular complexes, which have been interpreted as resulting from "ductular reduplication" in other studies of chronic pancreatitis, and "ductular proliferation" in studies of pancreatic adenocarcinoma, were studied carefully to determine their origin. Extensive retrogressive changes in acinar cells leading to diminished zymogen granules, decreased cell height, and concomitant increase in luminal diameter were consistent with the interpretation that phenotypic modulation of acinar cells to take on the characteristics of ductular cells produced the tubules. This it is concluded that in chronic pancreatitis, as has been shown for pancreatic adenocarcinoma, the tubular complexes originate from acinar cells rather than from proliferation of preexisting ductules. Fibrosis and thickening of the basal lamina of exocrine pancreatic cells and capillaries were consistent with an altered capability for transmission of material between blood vessels and exocrine cells.