Glucose tolerance and insulin action on lipogenesis in liver and both white and brown adipose tissues were studied in 70-day old rats undernourished from the fetal stage. Basal and glucose-induced plasma insulin levels were decreased in food-restricted rats without changes in glucose tolerance. Insulin injection caused a more sustained hypoglycemia in undernourished than in controls. The basal rate of lipogenesis, measured with tritiated water, was significantly higher in rats previously submitted to malnutrition. A hyperinsulinemic condition was established in the two populations of animals by means of the hyperinsulinemic-euglycemic clamp technique. Under this condition, lipogenic rate did not undergo a further increase in any of the tissues assayed from restricted rats whereas it increased in the controls, which is interpreted as if the metabolic pathway was already at the maximum rate in the basal state. These results, in conjunction with other previously reported, show that undernutrition leads to an increase in insulin action upon several target tissues. The precise causes for this change are not known at present.