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The aim of this study originally was to elucidate whether methylthiouracil will cause thyroid hyperplasia and inhibition of metamorphosis in frog tadpoles or not, provided it should prove to be sufficiently non-toxic to be employed experimentally. During the experiments, however, changes in the pigmentation of the larvae also occurred, and it is these observations which are chiefly described in this communication.

A study of the literature revealed some communications on related experiments. On the whole agreement exists that most of the substances which are known to be goitrogenic in mammals have the same effect in amphibian larvae [Gordon, Goldsmith & Charipper, 1943; 1945]. However, changes in pigmentation after such treatment are mentioned only by Lynn & De Marie [1946]. These investigators noted inhibition of metamorphosis in Rana sylvatica immersed into 0·05 % thiourea and thiouracil. Animals treated with thiourea did not develop further when reimmersed in water. From the 26th day