Blood serotonin concentration is implicated in the regulation of behavior as well as in the development of psychopathological symptoms. Serotonin transporter regulates the serotonin level by reuptaking it from the synaptic cleft. In this study, platelet serotonin concentrations and the serotonin binding constant (V_max) were compared in patients with schizophrenia and healthy individuals with different 5-HTTLPR genotypes. The study included 60 patients and 62 controls. The above biochemical parameters were associated with the 5-HTTLPR genotype. Both in patients and controls, carriers of the LL genotype had lower blood serotonin concentrations and V_max as compared to those with genotypes containing one or two S alleles. These results suggest that the 5-HTTLPR polymorphism of the serotonin transporter gene is involved in regulating the state of the serotonergic system.