We studied the effect of inhibition of platelet aggregation (obtained by omitting sample stirring and by the addition of ASA and GRGDS peptide) on ATP secretion and Ca2+movements. When collagen was the agonist, platelet aggregation stimulated release reaction and Ca2+movements in both the presence and absence of extracellular Ca2+. When platelets were stimulated by thrombin, ATP release and Ca2+movements were largely independent of aggregation. Our data suggest that platelet aggregation stimulates Ca2+movement, and that this phenomenon of feedback amplification of platelet activation plays an important role in platelet function when collagen is the agonist, while it has little or no role when thrombin is the stimulus.