Septic shock is a major cause of mortality and morbidity worldwide. Serotonin (5-HT) is released by activated platelets into the circulation, and is mediator of endothelial dysfunction. 5-HT metabolism is known in immune system via specific 5-HT receptor, also in effects on the peripheral nervous system. Kinetic of 5-HT, tryptophan, kynurenine, MAO activity and IDO activity in human septic shock was never investigated.