Disturbance of central serotoninergic system has been suggested in Down's syndrome (DS). In this syndrome the 5-HT concentration in blood platelets is below normal, but the mechanism behind this has been controversial. Recently, evidence has accumulated indicating a decreased active transport of 5-HT possibly due to lowered activity of Mg++-dependent Na+-K+-stimulated adenosine triphosphatase. In the present study the kinetic analysis of 5-HT uptake by blood platelets from DS patients revealed a decreased Vmax, suggesting decreased transport function but an unchanged affinity to the uptake receptors, as indicated by normal Km values. The controls were mentally retarded patients from the same institute. The uptake continued in a linear fashion up to 20 min., suggesting an unchanged storage of 5-HT. Also the effect of zinc on the 5-HT uptake was studied; plasma zinc levels have been noted to be lowered in DS infants. In vitro zinc caused a dose-dependent inhibition of uptake at 10(-5) and 10(-4) M. In vivo, after 2 weeks treatment 135 mg/day orally no significant effect was noted.