The separated plasma of gray seals (Halichoerus grypus) has an unusually high non-bicarbonate buffering capacity (beta(NB) d[HCO3-]/dpH = - 21 Slykes; Slyke = mmol l-1 pH unit-1). To examine whether this represents a trait of mammalian carnivora, or is related instead to deep diving, we examined blood from dog and killer whale, respectively. The beta(NB) of separated plasma of dog was typically low ( - 4 to - 8 Slykes), as in other mammals, whereas that of the whale ( - 13.5 Slykes), while significantly higher than dog, was not nearly as high as in seal. As one would theoretically predict of a blood system with exceptionally high extracellular buffering and functional anion exchange across the erythrocyte membrane, there was little difference in the buffer slope between true plasma and whole blood of seal. This appears to be a novel pattern of CO2 carriage in mammals. Erythrocyte ion concentrations were similar to other mammalian carnivora, with intracellular sodium concentration being high and potassium low. The red cell water content was unaffected by changes in oxygen saturation and tended to decrease when extracellular pH decreased below 7.4. Apart from the comparatively elevated Bohr factor, - 0.518 as a function of extracellular pH and - 0.550 as a function of red blood cell pH, the blood oxygen binding properties were similar to terrestrial mammals of similar size.