The ability of all eight D-aldohexose steroisomers to stimulate insulin release and biosynthesis was compared with their ability to serve as a metabolic substrate for isolated islets of Langerhans as judged by formation of lactate. Insulin release and synthesis were stimulated by glucose or mannose but not by allose, altrose, gulose, idose, galactose or talose. No potentiary effects of allose, altrose, gulose, idose, or talose were found on insulin release in the presence of 4 mmol/l glucose nor did these sugars inhibit insulin release in the presence of 20 mmol/l glucose. Lactate formation was increased above values found in the absence of added substrate by 20 mmol/l D-glucose or mannose, but not by allose, altrose, gulose, galactose or talose. The results support ]the substrate-site hypothesis for the recognition of sugars as stimuli of insulin release and synthesis.