The effect of somatostatin on plasma sugar and insulin levels was determined in 8 normal, 7 diabetic and 5 acromegalic subjects. An absolute fall in blood sugar values on the order of about 20% was noted, though during administration of the drug only, in all cases. Normal and diabetic subjects also showed an approximately 40% decrease in blood insulin, whereas no such fall was observed in the acromegalic. In spite of the simultaneous fall in insulin levels, it was clear that the reduction in blood sugar was ascribable to a drug-induced block of glucagon secretion. The part possibly played by somatostatin in the regulation of glucide homeostasis is examined in the light of these results.