SUMMARY: During the third part of pregnancy in the rat the concentrations of plasma insulin and tissue glycogen in the foetus increase progressively. These levels and the release of insulin by pancreas incubated in vitro were significantly higher than the values found in adult non-pregnant rats. After birth the correlation between plasma insulin concentration and the stores of glycogen was also evident. In the first day of life, the concentrations of plasma insulin, glycogen in liver, striated muscle and kidney decreased significantly; these values decrease even more during the first 15 days of lactation. After weaning started (20 days post partum), rapid increases of insulin and glycogen were observed, parallel to the slow growth of the newborn rat during the first 15 days of life and the more rapid rate of growth after 20 days. Similarly, the insulin content of the pancreas increased more significantly during the period in which pancreatic weight and plasma insulin concentrations increased more slowly. These results show that when plasma insulin concentrations increase, body growth and stores of glycogen are higher, suggesting an anabolic role of insulin in the foetal and newborn rat.