The uptake of serotonin ( 1 4 C-5-hydroxytryptamine, 5HT) in platelets and its kinetic characteristics were investigated in a group of women (n = 20) with probable Alzheimer's Disease (mean age = 76.0, years, SD = 8.27, range = 63-88) and in healthy normal women (n = 18, mean age = 72.6 years, SD = 7.24, range = 61-84). Both the apparent affinity of binding of 5HT to the platelet membrane (K m ) and the maximum velocity (V m a x ) of the rate of transport of 1 4 C-5HT into platelets were significantly higher in the Alzheimer's Disease group than in the normal comparison group. Initial velocity of 1 4 C-5HT uptake nor passive diffusion (nonspecific uptake of 1 4 C-5HT at 4°C) differed significantly in the two groups. Our findings suggest abnormalities in the kinetic mechanisms involved in the uptake of 1 4 C-5HT by platelets in women with Alzheimer's Disease.