Hyperpigmentation of the skin is described as a prominent feature of an infant born of a thyrotoxic mother with methimazol medication. Urinary excretion of dihydroxy-phenylalaine dopa or dopa-like substance was found, suggesting an evidence for an increased melanin synthesis in this case. A mechanism for an acceleration of the pathway tyrosine[forward arrow]dopa[forward arrow]melanin has been explained from the following two points: [long dash] (i) There was a high tyrosine level in serum. This finding associated with urinary output of p-hydroxyphenylpyruvic acid gave a strong evidence for the existence of a block of tyrosine-transaminase. An excessive amount of thyroxine transferred fromthe mother was presumably a factor responsible for the inhibition of tyrosine-transaminase in the fetal liver; (ii) There was a possibly increase of activity of the melanophore-stimulating hormone. This was probably caused by an excessive amount of thyroxine transmitted from the mother. This case may, within our literature survey, be the first case reported with such an anomaly.