The heat output of heparinized blood samples from seven mammalian species (hamster, rat, hedgehog, dog, swine, human and sheep) was determined by ampoule microcalorimetry and related to body mass. Typical microcalorimetric records consist of an initial plateau and a terminal decline of metabolic rate. A decrease in the plateau heat output with increasing body mass was found, corresponding to the allometric relationship between specific basal metabolic rate and body mass. Moreover, the duration of the plateau was nearly reciprocal to the level of heat output, indicating a relatively constant "energy content" of blood samples which in smaller species, due to their higher thermal power, is utilized faster than in larger ones. The presence of scaling effects in a physiological cell suspension may elucidate-the still unknown regulatory mechanisms of metabolic size allometry.