The allometric relationship between body mass and tissue metabolism was examined in liver and kidney cortex slices from mouse, rat, rabbit, sheep, and cattle, representing an approximately 12,000-fold difference in body mass and an 11-fold difference in mass-specific basal metabolic rate. Larger mammals have lower tissue metabolic rates (TMR; mumol O2.g wet wt-1.min-1) at 37 degrees C, yielding the equations TMR = 3.6 M-0.21 for liver slices and TMR = 3.2 M-0.11 for kidney cortex slices, where M is body mass in grams. This appears to be an intrinsic property of the tissue and is not due to differences in extracellular space or tissue protein content, because these are relatively constant in all mammalian species examined. The allometric relationships remain when tissue metabolism is expressed relative to "active cell mass" in tissue slices. Potassium uptake rate (KUR; mumol K+.g wet wt-1.min-1) was also measured (as 86Rb+ uptake) and was also lower in larger mammals, yielding the equations KUR = 1.2 M-0.14 in liver slices and KUR = 3.4 M-0.13 for kidney cortex slices. The energetic costs of sodium pump activity were estimated to be < 10% of TMR for liver and kidney cortex from all five mammalian species.