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The pathways by which cancer spreads are well known, but information concerning the factors that subsequently lead to the formation of secondary tumors is very scanty (1). The present paper deals with one such factor, namely temperature. Neoplasia, like normal biological processes (2), is greatly influenced by it, as shown in studies on the growth of intraocular transplants of frog carcinoma (3). Whether temperature also affects the complex process of metastasis is the principal question with which this paper is concerned.For studying the effects of temperature on metastasis the carcinoma arising in the kidneys of the leopard frog (Rana pipiens) has certain advantages:(a) It is representative of the commonest morphologic type of cancer in man and warm blooded vertebrates generally; i.e., the adenocarcinoma. It has the same structure, exhibits the same invasiveness of adjacent tissues, and possesses the same potentiality to form secondary tumors by metastasis (4). But unlike its counterpart in man and warm blooded vertebrates, this potentiality tends to remain unexpressed. (In this respect the frog carcinoma resembles many cancers of other cold blooded vertebrates.) The relatively low incidence of metastasis under natural conditions is favorable for experiments designed to induce or augment the process. (b) The frog carcinoma, originating in the kidney, often attains a size sufficient to allow its ready recognition in the living animal by palpation. The tumor may then be outlined more precisely by roentgenograms. Thus any experimentally induced change in size can be made apparent by periodic examinations.(c) Most important for the present experiments is the close dependence of the frog's internal temperature on that of the environment, and its ability to withstand relatively great changes for considerable periods. Thus an opportunity

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