Coefficients of temperature change in relation to body size are given for three species of fish native in Lake Huron, subjected to a sudden drop in ambient temperature of about 5C. These observations together with published data provide cooling coefficients for fish and aquatic reptiles over a weight range of 1–5000 g. The slope of the regression-log cooling coefficient on log body weight is about −0.6. The hypothesis offered is that the magnitude of the slope in question results from the temperature measured being close to the center of a body of constant thermal conductivity, retaining essentially the same proportions throughout growth.In addition, various observations of excess deep-body temperatures were collected in relation to weight for poikilothermal fish and aquatic reptiles, all presumed to be in thermal equilibrium with their environment. A double logarithmic plot of deep body temperature on weight in this case has a slope of about 0.4. The relation between these two slopes appears to be satisfactorily explained by the general observation that the slope of a double logarithmic plot of metabolic rate per unit weight on body weight has a slope of about −0.2.