The effect of muscarinic cholinergic receptor stimulation on pancreatic acinar cell division has been investigated. Muscarinic receptor stimulation, in addition to causing a 50% depletion of amylase in the pancreas after 4 h, resulted in a 400% increase in the incorporation of [6(-3)H]thymidine 27 h after drug administration. This increase in incorporation was dose-dependent. Autoradiography showed this increase to be due to an increased acinar cell labelling. Isoprenaline also increased the incorporation of [6(-3)H]thymidine into the pancreas but did not reduce enzyme levels. However, with isoprenaline there was no increase in acinar cell labelling. The possibility of a relationship between secretion and hyperplasia of acinar cells in the rat pancreas is discussed.