SUMMARYHarbor porpoises are active divers that exchange O 2 and CO 2 with the environment during a fast single breath upon surfacing. We investigated blood O 2 -transporting properties, buffer characteristics, Cl -transport via the erythrocyte anion exchanger (AE1), circulating nitric oxide metabolites and hemoglobin nitrite reduction in harbor porpoises with the aim to evaluate traits that are adaptive for diving behavior. Blood O 2 affinity was higher in harbor porpoises than in similar sized terrestrial mammals, as supported by our parallel recordings of O 2 equilibria in sheep and pig blood. Further, O 2 affinity tended to increase with increasing body mass. A high O 2 affinity favors O 2 extraction from the lungs, but a normal Bohr effect (logP 50 /pH-0.46) gradually lowers O 2 affinity during dives (where CO 2 accumulates) to assist O 2 off-loading to perfused tissues. The true plasma non-bicarbonate buffer value was moderately higher than in terrestrial mammals and increased upon deoxygenation. Plasma bicarbonate was also relatively high, contributing to increase the overall buffer capacity. The apparent Cl -permeability of harbor porpoise erythrocytes was similar to the human value at 37°C, showing absence of a comparative increase in the velocity of erythrocyte HCO 3 -/Cl -exchange to aid CO 2 excretion. The Q 10 for AE1-mediated Cl -transport in harbor porpoises was lower than in humans and seemed to match the Q 10 for metabolism (Q 10 Ϸ2). Plasma nitrite, plasma nitrate and hemoglobin-mediated nitrite reduction were elevated compared with mammalian standards, suggesting that increased nitric oxide bioavailability and nitrite-derived nitric oxide could play important roles in diving physiology.