Determinations have been made of the level of serotonin in various fetal and maternal tissues of goats and rabbits. In the goat, both fetal brain and blood were higher in serotonin content than comparable maternal tissues. Furthermore, in the goat fetal neocortical areas unexpectedly were found to be richer in serotonin than certain subcortical structures. In contrast to the goat, the rabbit was shown to have higher serotonin levels in maternal than in fetal blood. Moreover, when a large amount of serotonin was administered subcutaneously to pregnant rabbits, the fetuses began to die at a time when the maternal blood levels of serotonin had about doubled. This toxic action was shown to be due at least partially to the sensitivity of the umbilical vessels to the vasoconstrictor action of serotonin.