Four populations of 70-day-old undernourished rats were studied. All rats were submitted to a 65% restriction of food intake as follows: 1) undernutrition of the mother from day 14 of gestation until birth only (fetal period, group F), 2) undernourished during fetal and suckling periods only (group FS), 3) undernourished from day 14 of gestation until day 70 of life (group FSA), and 4) undernourished from day 28 of life until day 70 (group A). Body weight, plasma glucose, glucose tolerance, basal plasma insulin, in vivo glucose-induced insulin secretion, pancreatic insulin content, and in vitro response of isolated islets to glucose and amino acids were investigated. Islet insulin content decreased in all four groups studied, and islet mass was found reduced in group FSA. Glucose tolerance was decreased in group A, and in vitro response of islets decreased in both groups FSA and A. We conclude that two different adaptive processes should be considered: the possible long-term consequences of impaired pancreas development due to inadequate perinatal nutrition and the functional alteration of insulin secretion to undernutrition in adulthood.