Freshwater turtles, Chrysemys picta bellii, were submerged in groups of 7 at 3 degrees C in O2-free water for 1, 2, 4, 8 and 12 weeks. Blood samples from these turtles and from 10 normoxic turtles at 3 degrees C were analyzed for plasma concentrations of lactic acid, total CO2, Na+, K+, Cl-, Ca2+, total calcium, total magnesium and osmolality. Total lactate rose during anoxia to a mean peak value of 145 mM, but the decrease in HCO-3 and Cl- and increase in K+ balanced less than 40% of the lactate. Total calcium and total magnesium rose respectively by 9.5 and 6.0 times the normoxic values after 12 weeks, at which time free [Ca2+] was 25.0 mEq (37% of the total calcium). To evaluate the possible role of bound calcium in ion balance, test solutions with calcium, but with and without 145 mM lactate, were tested for free Ca2+. In the presence of lactate, over two-thirds of the total calcium combined with lactate- to form a calcium lactate complex (possibly CaLactate+). Based on these data, it is concluded that most of the bound plasma calcium in the anoxic turtles was combined with lactate. By assuming that magnesium reacts similarly with lactate, a complete account of plasma ion balance is accomplished and the turtle's plasma ionic response to extreme lactic acidosis is described. Plasma osmolality increased during anoxia by 100 mOsm and matched the mM rise in total measured and calculated ions.