Plasma glucagon levels rose within 1 hr after withdrawal of insulin in 7 juvenile-type diabetics previously kept normoglycemic by prolonged intravenous infusions of insulin. These changes preceded subsequent elevations in plasma alanine levels. Individual rises in plasma glucagon were correlated with elevations in plasma glucose, beta-hydroxybutyrate, free fatty acid, and glycerol levels, suggesting that glucagon may play an important role in the development of diabetic ketoacidosis in man.