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1. By density gradient centrifugation of homogenized blood platelets of guinea-pigs, subcellular organelles have been isolated which exhibit much higher concentrations (µmole/µg protein) of ATP than the whole platelets.

2. The organelles also store endogenous 5-HT whose concentration is, however, considerably inferior to that of ATP.

3. Investigated by electron microscopy most of the isolated organelles look empty and only very few show dense osmiophilic cores indicating the presence of 5-HT. Accordingly the occurrence of dense osmiophilic organelles in the whole platelets is also very rare.

4. Exogenous 5-HT taken up by the platelets markedly accumulates in the subcellular fraction containing the organelles storing ATP. Concomitantly, the platelets show a rise in the number of osmiophilic organelles and the frequency of osmiophilic cores in the isolated organelles is increased.

5. From these and previous in vitro experiments it is concluded that the so called 5-HT storage organelles of blood platelets primarily store ATP and only subsequently accumulate 5-HT.