A raised level of glucagon was attained rapidly, and maintained steadily, for an hour or more in the circulation of fed, or of fasted, rabbits. During this time the concentrations of 18 amino acids, of glucose and of insulin, were measured in samples of arterial blood and of blood leaving the skeletal muscles, taken simultaneously. The glucagon raised the level of glucose in the arterial blood, while, at the same time, decreasing the levels of most of the amino acids. The rate of release of amino acids from the skeletal muscles increased during this time. When the store of hepatic glycogen had been depleted by a previous injection of glucagon, or by fasting, glucagon still caused a rise in blood glucose, but the rise was less, and was less well sustained, than that seen when the glycogen stores of the liver were normal. The second injection of glucagon, or fasting, caused the glycogen depleted liver to convert certain amino acids, obtained from the blood, into glucose, lowering the blood levels of these amino acids. The muscles now released amino acids. There was no detectable difference in the release of amino acids from muscle whether glucagon was given systemically or into the artery supplying the muscles. However, a systemic injection of L-alanine, together with glucagon, abolished the fall in the level of amino acids in the blood, and suppressed their release from muscle. During fasting a steady fall in the blood levels of five amino acids occurred, probably due to their use by the liver for glucose synthesis; the temporary rise in the levels of other amino acids, which are not readily used for glucose synthesis, seems to be due to their concomitant release, from the breakdown of muscle protein. We conclude that the elevated level of glucagon, which is found in fasting, ensures that an acceptable level of the blood glucose is maintained by means of two mechanisms; first by releasing glucose from liver glycogen, and then, when fasting is prolonged, by causing the liver to synthesize glucose from certain of the amino acids in the blood, thus decreasing their concentration in the blood. This fall in the levels of these amino acids in the blood is, we believe, the stimulus which leads first to the release of amino acids from the muscles, and then to the breakdown of muscle protein to replace these released amino acids, and thus to maintain a continuous supply of these amino acids for glucose synthesis.