'-dibutyltryptamine, caused the rapid morphological change in platelets. In concentrations below those needed to produce the agonistic action (viz. 0.05-2.0 muM), these analogues themselves inhibited competitively the shape change caused by 5-HT.3. The velocity of change in shape caused by 5-HT was reduced in low Na media.4. Ten analogues produced platelet aggregation; three of these, viz. 5-methoxy-alpha