Pancreatic weight, [3H]-thymidine incorporation into DNA, labeling indices, and total DNA and RNA content were measured in rats treated with vehicle or 1 microgram/kg caerulein, 100 micrograms/kg secretin, or a combination of these peptides injected every 8 h for 1-5 days. Incorporation of [3H]thymidine into DNA increased 12-fold after 2 days of treatment with the combination of peptides. DNA content increased after 3 days and reached a level 1.8 times control after 5 days. Autoradiography showed that two cell types, acinar and an unidentified type, were the sites of increased DNA synthesis. Different patterns of labeling were seen in the two populations: acinar cell labeling indices were increased at 1 and 2 days (20-fold) and then fell; nonacinar cells showed an increase only after 2 days and maintained this increase after 5 days. Potentiation (greater than additive effects) was found when caerulein and secretin were injected together for all measurements except RNA content. These data indicate that DNA synthesis in two cell populations is affected by secretin and caerulein and support the occurrence of potentiation between secretin and caerulein for trophic effects on the exocrine pancreas.