This study examined the relative proliferation of the ductule cell compartment and the mononucleate and binucleate acinar cell populations in the developing pancreas in rats from 5 to 49 days of age. Proliferation of these cell types was assessed in the intact gland and in isolated acinar cells by autoradiography after in vivo labelling with tritiated thymidine at 5, 10, 17, 28, 35, 42 and 49 days of age. It was found that the acinar cell population was predominantly mononucleate at birth, but following weaning became progressively binucleate. At all times studied, DNA synthesis in mononucleate acinar cells was between 3- and 10-fold greater than in binucleate acinar cells. Ductule cell labelling was high relative to that seen in the adult from 5 to 17 days after birth, but after weaning duct cell labelling fell to levels seen in the adult. The results suggest that up to weaning acinus formation is derived from duct cell differentiation and mononucleate acinar cell proliferation, and that after weaning mononucleate acinar cells continue to replicate, either giving rise to binucleate acinar cells or continuing to divide as mononucleate cells. The mononucleate acinar cell thus appears to have the capacity to proliferate, while the binucleate acinar cell appears to be static and non-dividing.