Common map turtles (Graptemys geographica) were collected from a natural underwater hibernaculum in Vermont at monthly intervals during the winter of 1997-1998. Blood was sampled by cardiac puncture and analyzed for pH, PCO 2 , PO 2 , and hematocrit; separated plasma was tested for Na + , K + , Cl -, total [Ca], total [Mg], [lactate], and osmolality (mOsm kg -1 H 2 O). Control (eupneic; 1Β°C) values for pH, PO 2 , PCO 2 , [HCO 3 -], and [lactate] were 7.98 Β± 0.03, 47.4 Β± 18.7, 10.1 Β± 0.7 (mm Hg), 36.1 Β± 0.2 (mmol liter -1 ), and 2.1 Β± 0.1 (mmol liter -1 ), respectively. Between November 1997 and March 1998, ice covered the river and the turtles rested on the substratum, fully exposed to the water, and were apneic. Blood PO 2 was maintained at less than 3 mm Hg (range 0.9 Β± 0.2 to 2.1 Β± 0.7 mm Hg), PCO 2 decreased slightly, plasma [lactate] was <5 mmol liter -1 , and plasma [HCO 3 -] decreased significantly. In March [lactate] rose to 7.5 Β± 1.5 mmol liter -l , but there was no acidemia. Map turtles meet most of their metabolic demand for O 2 via aquatic respiration and tolerate prolonged submergence at 1Β°C with little change in acid-base or ionic status. The adaptive significance of remaining essentially aerobic during winter is to avoid the life-threatening progressive acidosis that results from anaerobic metabolism.