Caerulein-induced pancreatitis (CIP) in rats is characterized by oedema and cell necrosis followed by spontaneous regeneration. The ras protein as well as ornithine decarboxylase (ODC) play a central role in the transmission of signals induced by growth factors. Therefore, we analyzed these gene products during the course of CIP and during the regeneration of the gland. Growth and biochemical parameters (pancreatic weight, total DNA, RNA and proteins) were determined along with ODC activity and quantitative reverse-transcrip-tase polymerase chain reaction measurements of mRNA levels. During CIP, the significant increases in pancreatic weight were the result of oedema. During that period, maximal increases in ODC activity were observed at 3 h, in ODC mRNA expression at 2, 3, and 4 h, and in Ki-ras mRNA expression at 1 h. During the 3-day resting period within which no treatment was given, pancreatic weight exhibited its maximal reduction after 2 days in the CIP group. In that same group, the ODC activity reached its maximal level above control after 3 days and ODC and Ki-ras mRNA expression after 1 and 2 days. During the regeneration period of 5 days, the pancreata of the untreated pancreatitis rats did not totally recover, whereas those of the animals receiving the small dose of caerulein (1 μg) showed full recovery and even a significant increase above control after 5 days. During that period, maximal increases in ODC activity and Ki-ras mRNA expression occurred after 1 day of caerulein treatment; ODC mRNA expression was also significantly increased after 3 and 5 days in the pancreatitis animals with no effect of caerulein treatment. The positive effect of caerulein on Ki-ras mRNA suggests that the cholecysto-kinin analogue can induce the expression of essential growth-promoting genes