It has long been speculated that urea accumulated during seasonal dormancy contributes to metabolic depression. Recent work suggests urea can indeed act as a metabolic depressant during dormancy in a number of taxonomically diverse species of ectotherms. The mechanisms by which urea exerts its hypometabolic effect are unknown, but potentially stem from inhibition of mitochondrial respiration. We isolated mitochondria from Rana sylvatica skeletal muscle, an organ that is metabolically responsive to urea, and measured respiration rates in the absence or presence of 80 mmol l(-1) urea in the respiration medium. Because the effect of urea may be influenced by the intracellular milieu, in these experiments we varied substrate (pyruvate or palmitoylcarnitine), temperature (4, 10, or 15°C), and pH (6.8 or 7.4). Oxygen consumption of control and urea-treated mitochondria was sensitive to each of these variables, but neither state 3 nor state 4 respiration was reduced by urea treatment and, to the contrary, urea treatment slightly increased state 4 respiration at higher test temperatures. Although we did not test the efficacy of other incubation times or urea concentrations, the outcome of our experiment intimates that the urea-induced hypometabolism observed in hibernating R. sylvatica results from inhibition of energy-utilizing processes elsewhere in the cell, rather than a direct inhibition of mitochondrial respiration. Future investigation into urea's effects on non-mitochondrial metabolic pathways is necessary to uncover the mechanisms by which urea depresses metabolic rate.