[en] Effects of various experimental conditions on serotonin (5-HT) uptake in human platelets were examined. The experimental design allowed the evaluation of the effect of diffusion and other non-saturable processes on the affinity and maximum activity of the membrane pump for 5-HT uptake. Total 5-HT uptake was determined by incubating platelet-rich plasma (PRP) with increasing concentrations of serotonin at 37⁰C for 4 min. The passive uptake was measured by the addition of various 5-HT concentrations to PRP in buffer at 37⁰C, followed by immediate transfer to an ice-cold water bath. The difference between the total and passive uptake was linear for 6 min. The affinity (Ksub(m)) for active platelet serotonin uptake was 0.45 +- 0.09 μmol/l and maximal rate of uptake (V) was 10.7 +- 2.1 pmol/10⁷ platelets/min. The described method provides a convenient and reliable measure of active 5-HT uptake suitable for clinical investigation. The effect of passive diffusion on kinetic parameters is discussed. (Auth.)