5-Hydroxytryptamine (5-HT) will interfere with pregnancy in mice at all stages, resulting in death of the foetuses (Poulson, Botros & Robson, 1960). The mechanism by which the drug acts in the second part of gestation, when it usually kills the foetuses within 15–60 min. of administration to the mother, has been investigated and the following possibilities are considered: (1) 5-HT exerts a direct toxic effect on the foetuses. Its concentration in the foetuses has been determined following an injection into the mother of a dose of 0·5–1·5 mg. 5-HT which is rapidly lethal to the foetuses, but is without any obvious acute toxic effect on the mother. The foetal content of 5-HT 1 hr. after injection into the mother was found to be 0·117 μg./g. (average of foetuses from 16 mice), and by this time all of the foetuses were dead. In contrast, direct injection of 1–5 μg. of 5-HT