SUMMARY1. Serotonin (5‐hydroxytryptamine) contents of maternal blood, fetal cord blood, and placenta from thirty‐five patients with toxaemia of pregnancy and forty normal pregnant subjects were estimated. Placental monoamine oxidase activity from ten normal and ten toxaemic cases were also estimated.2. An increased content and uptake of serotonin by platelets in maternal blood from toxaemic patients were observed and there was a concomitant reduction in platelet count.3. Raised levels of serotonin and a decrease in monoamine oxidase levels were observed in placentas of the toxaemic group.4. The levels of serotonin in fetal cord blood were significantly higher than those in normal maternal blood. It is suggested that the fetus could be the source of increased serotonin in this syndrome, due to the decrease in monoamine oxidase activity in the toxaemic placenta.5. An abnormality in indole amine metabolism may contribute to the pathology associated with toxaemia of pregna