Kinetics of 5-HT uptake by blood platelets was studied om eleven patients with Huntington's chorea and in ten patients with presenile dementia of Alzheimer type. In both groups of patients 5-HT uptake was unchanged in comparison to that of respective controls of the same age. The results do not confirm earlier reports of an increased 5-HT uptake by blood platelets in Huntington's chorea. Platelet 5-HT uptake does not seem to serve as biological test in either disease.