Insulin-like growth factors (IGFs) are important peptides involved in the regulation of cell growth and differentiation in many tissues. The ontogeny of IGF-I was examined in pancreata from 19-day rat fetuses, newborns and 5-, 11-, 26- and 70-day-old rats. For the regeneration studies two models were used: (i) 90% pancreatectomy was carried out and the rats were killed at 1, 2, 3 and 6 days after resection; (ii) acute pancreatitis was induced with caerulein (12μg/body weight three times a day every 8 h for 2 days) and the rats were killed at 1, 2, 5, 7 and 9 days after the first injection. Total RNA was extracted by the guanidinium isothiocyanate method and Northern blots were performed using total RNA and labeled cRNA probes. Abundance of the different mRNA transcripts was estimated by densitometric scanning and normalized to the abundance of 18 S rRNA for each time point. Northern blot analysis during ontogeny showed four (0·8–1·2, 1·9, 4·7 and 7·5 kb) major transcripts in the rat pancreas and liver. Total IGF-I mRNA was 40-fold higher in the adult liver than in the adult pancreas. Moreover, in the liver, IGF-I mRNA levels were higher in the adult than in the fetus, whereas in the pancreas, the highest levels were observed around birth. During the first 3 days after pancreatectomy, a peak of maximal expression was observed after the second day. Densitometric analysis of each IGF-I mRNA species showed concomitant increases in all transcripts. After 6 days, all transcripts had returned to near-control values. IGF-I mRNA expression 2 days after pancreatectomy was 3·5-fold higher than in the newborn. During the first 2 days of acute pancreatitis induction, overexpression of IGF-I mRNA was observed. However, soon after the second day of caerulein treatment, the 7·5 kb transcripts remained elevated whereas those of the others regressed toward control values. Our results show that IGF-I mRNA is overexpressed in both models of pancreatic regeneration but downregulated in the normal adult pancreas.