Ammocoetes of Lampetra were found to display a diurnal color change rhythm which diminishes in intensity as the ammocoetes grow older and changes character after metamorphosis when the animals develop lateral eyes. The color change rhythm in the ammocoete can be abolished by hypophysectomy or pinealectomy and, in the metamorphosed animals, by hypophysectomy or removal of the lateral eyes. The pineals of paling ammocoetes contain a substance which is capable of aggregating the pigment in the melanophores of ammocoetes and of Xenopus laevis tadpoles (the latter being far more sensitive). The paling substance is very similar in action to the mammalian pineal hormone, melatonin, but synthetic melatonin, itself, has only a slight effect on ammocoete melanophores. However, melatonin was shown to have an antagonistic effect when administered to hypophysectomized ammocoetes in conjunction with melanophore-stimulating hormone.