Serotonin metabolism in the blood and placental homogenate was studied in pregnancy aggravated by exacerbation of herpesvirus infection. Blood serotonin content in pregnant patients increased with increasing herpesvirus antibody titers and reduction of monoamine oxidase activity. Urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA) decreased. The decrease of monoamine oxidase activity in placental homogenate led to an increase of serotonin level, which was paralleled by destruction of mitochondria in the syncytiotrophoblast, suppression of BCL-2 protein activity, and increase of caspase-3 level.