Tiger salamanders (Ambystoma tigrinum) showed significant routine daily cycles of oxygen consumption at 15 and 25 C. Routine blood lactate and blood glucose values at 15 C showed no significant variation over a 24-h cycle. Aerobic scope during 30 min of forced activity was 30.0 and 33.8 µl g⁻¹ h⁻¹ at 15 and 25 C, respectively. Anaerobiosis was the primary source of energy production during activity; 61.4% and 66.5% of the total ATP produced at 15 and 25 C, respectively, was derived anaerobically. Correlation of O₂ debt with lactate production suggests that the oxygen debt is primarily lactacid. The volume of the O₂ debt would allow gluconeogenesis of only about half of the total lactate formed, the remainder presumably being otherwise eliminated. Liver lactate concentrations returned to resting levels before blood levels, an indication that other organs (nongluconeogenic) function in lactate removal.