1. The distribution of 14C-5-HT following infusion is different from that of endogenous serotonin. One hour after infusion it is, in major part, a function of the distribution of blood platelets.

2. The spleen in rats is the site of a pool of platelets. Based on both 14C and platelet recovery data in normal and splenectomized animals, this pool approximates 40 per cent of the total circulating platelets.

3. Postsplenectomy thrombocytosis may relate to removal of a platelet reservoir with shift of the platelets normally contained therein to the peripheral circulation.

4. The shape of platelet survival curves in rats is neither strictly linear nor curvilinear but normally is determined primarily by age-related processes in the platelet. Platelet survival curves tend to become curvilinear in thrombocytopenic animals, indicating that there is probably an additional small, fixed random loss of platelets from the circulation.

5. The disappearance of 14C-5-HT from blood platelets approximates but is not completely representative of the disappearance of the platelets themselves. It is probable that elution from platelets and possibly reutilization of platelet label occurs.

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