Aggregation induced by adenosine diphosphate is inhibited by adenosine as well as by adenosine mono- and triphosphate. Thrombin-induced aggregation is also inhibited by adenosine and the monophosphate, but the triphosphate at a similar concentration is not inhibitory. Aggregation induced by tri-ethyl tin is accelerated by a critical concentration of triphosphate and delayed by a higher concentration. The tri-ethyl tin effect is inhibited by 5-hydroxytryptamine and by tryptamine. Platelets from patients treated with reserpine respond normally to these last two substances. Incubated platelets lose first their ability to respond normally to tri-ethyl tin, then to thrombin, and lastly to adenosine diphosphate. These results are briefly discussed.