In this procedure, modified from Lundin et al. (1), the firefly luciferin-luciferase system is used to produce a light output proportional to the ATP concentration in the extracellular medium from platelet preparations. This output is measured in a luminometer to estimate the ATP concentration. The subsequent addition of pyruvate kinase/phosphoenolpyruvate (PK/PEP) converts the ADP present to ATP, allowing estimation of ADP from the increase in light output. The further addition of adenylate kinase/CTP converts the AMP present to ADP, which is transformed into ATP by the PK/PEP and estimated from the further increase in light output (2). The detection limit is approx 20 nM for each nucleotide in the original preparation. The range is up to approx 10 μM total nucleotides. This technique therefore provides a sensitive method for the measurement of the secretion of ADP and ATP from platelet dense granules.