Summary1. The specificity of platelet receptor sites for 5-HT uptake and for the rapid morphological change and aggregaition was investigated with 5-hydroxytryptamine (5-HT) and seventeen analogues as well as with some antagonists of 5-HT.2. The analogues, with the exception of 5-hydroxy-N'N'-dibutyltryptamine, caused the rapid morphological change in platelets. In concentrations below those needed to produce the agonistic action (viz. 0-05-2-0 )L,M), 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-methoxya-methyltryptamine, 5-hydroxy-a-methyltryptamine and 5-hydroxy-N'N'-diisopropyltryptamine), were approximately equipotent with 5-HT. Six analogues did not induce platelet aggregation. 5. All the analogues which prevented the initial change in shape of platelets caused by 5-HT also inhibited its aggregating effect, apparently competitively with low Ki values (0-02-1-6 juM).6. As with the inhibition of shape change, the inhibition of aggregation shows relatively low structural specificity of the receptor site. 7. Methysergide was a potent inhibitor of shape change and aggregation (Ki-0-03 /LM); imipramine was much less inhibitory (Ki-5-10,uM).8. Only one analogue (5-hydroxy-a-methyltryptamine) was taken up like 5-HT by platelets. All the other analogues inhibited the uptake of 5-HT by platelets (K=0-2-2-7 gum). 9. Methysergide was a weak inhibitor of 5-HT uptake (Ki-125 ,M) whereas imipramine was very effective (Ki~-0-3 uM).10. Our results show that the initial change in shape of platelets is required for and precedes aggregaition. The structural specificity of the platelet receptor concerned with shape change and aggregation caused by 5-HT appears low whereas the uptake mechanism is a highly specific one. The uptake probably proceeds through more than one step, the relationship between the steps is not yet clear.

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