It was suggested by Brandt in 1936 that the placenta exerts an inhibitory effect on thyroxine circulating in the maternal and foetal blood. Experiments are now reported confirming this suggestion. In feeding experiments with amphibia it has been found [Brandt, 1935] that thyroxine inhibits growth and increases the rate of metamorphosis, whilst placenta has a precisely opposite action, namely, that it increases growth but delays metamorphosis. Axolotls are so sensitive to thyroxine that even small doses of that substance will induce in these animals changes (alteration of body proportions, shrinking of gills and loss of weight) which can be accurately determined and which may, therefore, be used as a measure of the effect of the thyroxine. For comparative experiments of this kind one of the authors [Brandt, 1933] has already pointed out that it is necessary to use 'animals of equivalent constitution', i.e. of the same stock and of the