Endogenous CCK release induced by a synthetic trypsin inhibitor, camostat, stimulates pancreatic growth; however, the mechanisms mediating this growth are not well established. Early response genes often couple short-term signals with long-term responses. To study their participation in the pancreatic growth response, mice were fasted for 18 h and refed chow containing 0.1% camostat for 1-24 h. Expression of eighteen early response genes were evaluated by quantitative PCR; mRNA for seventeen of the eighteen increased at 1, 2, 4 or 8 h. Protein expression for c-jun, c-fos, ATF-3, Egr-1 and JunB peaked at 2 h. Nuclear localization was confirmed by immunohistochemistry of c-fos, c-jun and Egr-1. Refeeding regular chow induced only a small increase of c-jun and none in c-fos expression. JNKs and ERKs were activated 1 h after camostat feeding as was the phosphorylation of c-jun and ATF-2. AP-1 DNA binding evaluated by EMSA showed a significant increase 1-2 h after camostat feeding with participation of c-jun, c-fos, ATF-2, ATF-3 and JunB shown by supershift. The CCK antagonist IQM-95,333 blocked camostat feeding induced c-jun and c-fos expression by 67 % and 84 % respectively and AP-1 DNA binding was also inhibited. In CCK-deficient mice, the maximal response of c-jun induction, of AP-1 DNA binding was reduced by 64 % and 70 % respectively. These results indicate that camostat feeding induces a spectrum of early response gene expression and AP-1 DNA binding and that these effects are mainly CCK dependent.