A series of 56 normal rats and a series of 14 starved rats were weighed, measured, and their surface areas determined by measuring a collodion film which had been applied to the depilated skin. The data obtained were used to construct a formula for the prediction of the surface area of the rat. The best results were obtained by using the formula S = 7.47W2/3 for the normal animals and S = 7.64W2/3 for the starved ones. The standard deviations of the percentage errors of the above are 4.84 per cent and 4.74 per cent respectively. By using the constants given above the heat production of the rat referred to the surface area is more comparable to that of other animals than when older constants were used in the area prediction.