The aggregation of human platelets after addition of arachidonic acid (AA) is accompanied by a 30 fold increase in the net flux through the hexose monophosphate shunt (HMPS). The levels of reduced glutathione (GSH) and NADPH as well as the NADPH/NADPH+NADP quotient show a temporary fall which is restricted to the interval between AA addition and beginning of aggregation suggesting a lag phase between the onset of enhanced coenzyme consumption and that of increased coenzyme regeneration. together with literature data the results point to a possible regulatory function of reduced coenzymes and the HMPS in the process of platelet activation.