We investigated serotonin (5-HT) metabolism in the fetus in preeclampsia by measuring free 5-HT, 5-hydroxyindoleacetic acid (5-HIAA) and tryptophan concentrations in umbilical cord plasma and amniotic fluid, and 5-HT content in platelets by high performance liquid chromatography with electrochemical detection. Free 5-HT and 5-HIAA levels in umbilical cord plasma were significantly higher in the severe preeclamptic group (S) than those in the non-preeclamptic group (N). The plasma 5-HIAA/5-HT ratio in group S was significantly lower than that in group N. Platelet 5-HT content in group S tended to be lower than that in group N. Mean plasma beta-thromboglobulin concentration in group S was significantly higher than that in group N. There was no significant difference in plasma tryptophan levels between group S and N. These findings suggest that the higher levels of plasma free 5-HT in umbilical cord plasma in preeclampsia are attributable to excess release of 5-HT from platelets as well as lower monoamine oxidase activity.