We examined changes in blood gases, plasma ions, and acid-base status during prolonged submergence (6 h) of four aquatic turtle species in aerated water at 20 degrees C. Our objective was to determine whether the temperate species, Chrysemys picta bellii, exhibits greater tolerance to submergence apnea than the tropical species, Pelomedusa subrufa, Elseya novaeguineae, and Emydura subglobosa. Blood was sampled from indwelling arterial catheters for measurements of blood PO2, PCO2, pH, and hematocrit and for plasma concentrations of lactate, glucose, Na+, K+, Cl-, total Ca, and total Mg. The pattern of change was similar in all species: a combined respiratory and metabolic acidosis associated with a marked decrease of blood PO2. The severity of the acidosis developed in the temperate species, however, was significantly less than that of the tropical turtles. Lactate rose significantly and HCO3- fell proportionately in all turtles; changes in other plasma ion concentrations were small but were generally in the directions consistent with compensatory exchanges with other body compartments; i.e., cations (K+, Ca, and Mg increased) and anions (Cl- decreased). The results indicate that hypoxia tolerance is a conserved trait in turtles, even in those that do not experience enforced winter submergence, and that the temperate species may be superior in this capacity because of reduced metabolic rate.