Pancreatic acinar cells from rats 5 to 658 days (94 weeks) of age were isolated by enzymatic dissociation and stained with the DNA specific fluorochrome Hoechst 33258. The nuclear DNA content and the incidence of binucleation were estimated in these cells. Total pancreatic weight, RNA, protein and DNA, and the incorporation of 3H-thymidine into pancreatic acinar cell DNA were also estimated in similar animals as measures of pancreatic growth. From 5 to 17 days after birth, 95% of the cells were mononucleate diploid and 5% were binucleate diploid; but during the period of rapid pancreatic growth over the following 39 days, acinar cells became increasingly binucleate. By 56 days after birth, 64% of cells were binucleate with a diploid DNA content per nucleus; and the incidence of binucleation then remained constant. At 28 days of age, 4% of mononucleate cells were tetraploid, increasing to 6% at 658 days of age. At this time 3% of binucleate cells contained dual tetraploid nuclei. There is thus a rapid development towards diploid binucleate acinar cells in the growing, postnatal pancreas; and in the adult pancreas a small proportion of these cells develop tetraploid nuclei.