The uptake of thymidine-H(3) in the acinar nuclei of the rat pancreas was studied by autoradiography after segmental excision of the pancreas, ligation of a segmental pancreatic duct, or ligation of the artery to the splenic segment of the pancreas. No sustained increase of nuclear labelling occurred in the rest of the pancreas after resection of the splenic segment. Ligation of the artery to the splenic segment produced significant increase of labelling three to five days post-operatively. Ligation of the duct to the splenic segment caused a similar significant increased labelling in acinar cells which were degenerating. The cells and nuclei were smaller than normal and there was a decrease of basophilia. The increase was of the same degree seen after ethionine necrosis. Decrease of intranuclear DNA-controlling influences, such as a histone, or the increased availability of DNA precursors might be responsible for this phenomenon.