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1. Blood platelets from normal children and children with the trisomy 21 form of Down's syndrome (mongolism) were studied to determine the cause of the well established reduction in platelet 5-HT in the disease.

2. Concentrations of endogenous 5-HT in the platelets from mongols were 25·3% of the concentrations found in normal children.

3. The net accumulation of 5-HT in the mongol cells was decreased to 52·7% of normal. This reduction was probably due, in part, to a defect in 5-HT transport, because the initial rates of 5-HT uptake at plasma concentrations of 10^-6 and 10^-5 M were significantly slower.

4. Experiments on the efflux of 5-HT from mongol platelets loaded with amine showed that the rate of loss was initially 2·6 and later 7·8 times faster than normal.

5. Platelet ATP in mongol cells was 26% of normal, and the reduction of ATP and 5-HT was in the molar ratio of 3:1.

6. It is considered that the low platelet 5-HT in Down's syndrome is due to a defective 5-HT transport mechanism and impaired 5-HT binding, resulting from a reduction in the essential binding substance, ATP.