Washed human platelets have been labeled with either 32Pi or glycerol-1-14C and the distribution of the label in the phospholipids determined. 32Pi was introduced primarily into polyphosphoinositides, i.e. di- and triphosphoinositide, whereas the label from glycerol which indicates de novo synthesis of lipid molecules did not appear in these phospholipids. In the course of thrombin-induced aggregation and release the phosphate incorporation into phosphatic acid, di- and triphosphoinositide was rapidly stimulated in parallel to the platelet reaction. The incorporation of glycerol did not change under the same conditions. It is concluded that phosphoinositides with rapid incorporation of phosphate groups are not as rapidly synthesized de novo and presumably form a separate phospholipid pool in the platelets. Only the phosphorylating reactions are stimulated by the thrombin aggregation. The necessary enzymes for these reactions, namely diglyceride kinase, phosphatidylinositol kinase, and phosphatidylinositol-phosphate kinase all can be shown to be associated with a well characterized platelet membrane fraction.