A programmed turnover of pancreatic beta cells occurs in the neonatal rat involving a loss of beta cells by apoptosis, and their replacement by islet cell replication and neogenesis. The timing of apoptosis is associated with a loss of expression of a survival factor, insulin-like growth factor-II (IGF-II), in the pancreatic islets. Offspring from rats chronically fed a low protein isocalorific diet (LP) exhibit a reduced pancreatic beta cell mass at birth and a reduced insulin secretion in later life. This study therefore investigated the impact of LP on islet cell ontogeny in the late fetal and neonatal rat, and any associated changes in the presence of IGFs and their binding proteins (IGFBPs). Pregnant Wistar rats were fed either LP (8% protein) or normal (C) (20% protein) chow from shortly after conception until the offspring were 21 days postnatal (PN). Bromo-deoxyuridine (BrdU) was administered 1 h before rats were killed and pancreata removed from animals between 19.5 days fetal life and postnatal day 21. Offspring of rats given LP diet had reduced birthweight, pancreatic beta cell mass, and pancreas insulin content, with smaller islets compared with control fed animals, which persisted to weaning. Histological analysis showed that islets from pups given LP diet had a lower nuclear labeling index with BrdU in the beta cells, although, paradoxically, more beta cells showed immunoreactivity for proliferating cell nuclear antigen (PCNA). Because PCNA is present in G1 as well as S phase of the cell cycle, we quantified the number of beta cells immunopositive for cyclin D1, a marker of G1, and NEK2, an indicator of cells in G2 and mitosis. More beta cells in islets from LP-fed animals contained cyclin D1, but less contained NEK2 than did those in controls. This suggests that the beta cell cycle may have a prolonged G1 phase in LP-fed animals in vivo. Offspring of rats given C diet had a low rate of islet cell apoptosis detected by the TUNEL method in fetal and neonatal life (1-2%), with a transient increase to 8% at PN day 14. Offspring of rats receiving LP diet demonstrated a significantly greater level of islet cell apoptosis at every age, rising to 15% at PN 14. IGF-II mRNA was quantified in whole pancreas and was significantly reduced in LP-fed animals at ages up to PN day 10. IGF-II immunoreactivity within the islets of LP-fed rats was also less apparent, but no changes were seen in immunoreactive IGF-I or IGFBPs-2 to -5. These findings show that LP diet changes the balance of beta cell replication and apoptosis in fetal and neonatal neonatal life, which may involve an altered length of beta cell cycle, and contribute to the smaller islet size and impaired insulin release seen in later life. A reduced pancreatic expression of IGF-II may contribute to the lower beta cell proliferation rate and increased apoptosis seen in the fetus and neonate after feeding LP diet.