In vitro investigations were carried out on the action of insulin, glucose, xylose, galactose, fructose and sorbitol on the platelet aggregation test according to Breddin, on the ADP- and collagen-inducced aggregation and the release of platelet factor 4 as well as on the retraction. When incubating platelet-rich plasma with insulin and glucose simultaneously, a marked inhibition of the ADP- or collagen-induced platelet aggregation and release reaction results. Insulin as well as glucose impaired the platelet function only at high concentrations, this inhibition however did not reach that of a combination of both. Fructose, xylose and sorbitol exerted no significant inhibitory effects. In contrast to the prevented aggregation, the retraction was enhanced. As the causal mechanism for the inhibition of the platelet function the Crabtree effect is discussed.