In this relatively rapid liquid-chromatographic procedure the endogenous serotonin in blood platelets is quantified by fluorometry. We used this experimental procedure to estimate the platelet-release reaction as an indicator of the functional integrity of stored platelets. a decrease in the platelet-release reaction more sensitively indicates platelet changes during storage than do alterations in total ATP concentration or adenylate energy charge. The described method for quantifying serotonin is convenient enough for routine use in blood banks.