Rates of oxygen consumption and carbon dioxide release were measured in calling and resting European tree frogs using open-flow-through respirometry. The energetic cost of calling was high with an average of 1.076 ml O2/(g.h) at average call rates of 8,000 calls/h. The maximum factorial metabolic scopes averaged 24 with momentary peak values ranging between 5 and 41. There was a threefold difference in O2-consumption between individual males calling at the same rate. Respiratory quotients indicated that both lipids and carbohydrates were used to fuel calling. Carbohydrates provided the major fuel (69% on average) with dependence on carbohydrates increasing with call rate. In contrast to marathon runners, there was no shift in metabolic substrate use over a calling period of 2-3 h.