T he mechanism of stimulus-response coupling in human platelets was investigated with a new instrument that simultaneously monitors aggregation and secretion in the same sample of plateletrich plasma. When platelets were stimulated by high concentrations of ADP, secretion began only after aggregation was almost complete. With lower concentrations of ADP or with epinephrine, biphasic aggregation was observed, and secretion began simultaneously with, or slightly after, the second phase of aggregation. When platelets were stimulated with high concentrations of γ-thrombin or A23187, secretion and aggregation began essentially together. With very low concentrations of γ-thrombin or A23187, biphasic aggregation was observed with secretion paralleling the second phase. At every concentration of collagen, secretion and aggregation appeared to be parallel events. Under every condition where the beginning of secretion lagged behind aggregation, secretion was dependent upon aggregation and was inhibited by indomethacin; this is referred to as aggregation-mediated platelet activation. When secretion began at the same time as aggregation, it also occurred in the absence of aggregation and was not blocked by indomethacin; this is referred to as directly induced platelet activation. These observations are consistent with a simple model of platelet stimulus-response coupling that includes two mechanisms for activation; aggregation-mediated activation is inhibited by indomethacin, while direct activation does not depend upon aggregation and is not inhibited by indomethacin. Secretion and second wave aggregation appear to be parallel events, with little evidence for second wave aggregation being a consequence of secretion as usually described.