In fed, unanesthetized rabbits, regular zinc insulin, 50 U./kg. intravenously, decreased plasma glucose levels 52 per cent, p = 0.002, 35 minutes after injection. In 15-hour-fasted, unanesthetized animals, the same dose of insulin decreased plasma glucose levels 68 per cent, p less than 0.001. Plasma K+ concentration was not affected by insulin injection in the fed animals; in fasted rabbits, plasma K+ levels fell 26 per cent, p = 0.006. Despite this unequivocal evidence of insulin action in both sets of animals, there was no change in the K+, Na+, or H2O content in the brains of the same animals 35 minutes after insulin injection. These results, which give no evidence of a direct effect of insulin on electrolyte transport in brain, are in sharp contrast with those found in anesthetized rabbits, which suggested that insulin affects brain potassium and water content before any change in plasma glucose occurs.