From homogenized platelets from pig blood a fraction (P 1), was obtained through centrifugation at 3000 g (30 min) which contained 86 per cent of the structurebound ATP of the homogenate. Aliquots of this fraction were incubated in 30 min at 0° or 37° with and without various additives and then separated into sedimentable and soluble parts. There followed a determination of the stationary concentrations of ATP, ADP and AMP in sediment and supernatant of the fraction. The effect of these adenine nucleotides of various buffer systems, cations, SH-group inhibitors, cysteine, inhibitors of oxidative phosphorylation, EDTA, amine liberators, proteases, membrane-damaging substances, as well as osmolysis was investigated. Substances which lower the ATP level in the sediment were seen to produce the same effect on ADP and, with the exception of cysteine, on AMP (56 mM K +, cysteine, mersalyl, monoiodoacetic acid, NEM, reserpine 10 −3 M, tyramine, prenylamine, Triton, lipase and phospholipase D as well as osmolysis). With the exception of NEM, reserpine, phospholipase D and osmolysis, these substances lead to an accumulation of ATP in the supernatant of P 1. Other substances raise the ATP content in the supernatant of P 1 but did not lower the stationary ATP concentration in the sediment (Ca 2+, EDTA, chlorpromazine and pheoxybenzamine). Substances which cause a higher concentrations of ATP in the supernatant of P 1 show the same effect on ADP. An accumulation of ATP in the supernatant is followed by a sinking of the AMP content. Exceptions are potassium (56 mM) and tyramine, whereby the AMP concentration rises, as well as Triton. Weak effects on the structure-bound nucleotides were observed after addition of calcium, magnesium, phenoxybenzamine and sodium azide. As the concentrations used Na 2+, 2·4 dinitrophenol, the ferments trypsin, thrombin and lysozym, and ouabain failed to influence the examined parameters. The results obtained with fraction P 1 show an ATPase activity, which may be related to the liberation of structure-bound adenine nucleotides from blood platelets. The effect in this adenine nucleotide pool of various substances conform only partially to amine-containing organelles in other cells.