Oxygen consumption of calling male spring peepers (Pseudacris crucifer) was measured at five temperatures (7, 10, 15, 19, and 23°C) encompassing the natural range of variation encountered during the breeding season. Oxygen consumption increased as a linear function of calling rate, which in turn was a linear function of temperature. Hence, nearly all the increased cost of calling at warmer temperatures was accounted for by increases in calling rate; temperature did not appear to have any independent effect on activity metabolism. During one breeding season, ambient temperature increased throughout the season, resulting in an hourly cost of calling at the end of the season that was double the cost at the beginning. Periodic warm spells also resulted in major increases in the cost of calling.