KOLKHORST, F. W., T. D. TOEPFER, and F. A. DOLGENER. Expired air temperature during steady-state running.Med. Sci. Sports Exerc., Vol. 27, No. 12, pp. 1621–1625, 1995. While some metabolic measurement systems measure expiratory temperature to standardize gas volumes, other systems use only an estimate. This study investigated the effect of prolonged exercise on expiratory temperature near the pneumotachometer to provide a basis for its estimation when actual measurement is unavailable. Seven active females each performed two 45-min treadmill runs at identical speeds (64.5% ± 11.8% of &OV0312;O2max) in which the pneumotachometer heater control was either set to 37°C or turned off. Expired air temperatures were monitored with thermocouples at the nonrebreathing valve (VAL) and 1 cm upstream (UPS) and downstream (DNS) from the pneumotachometer screens. There were no temperature differences over time for any of the conditions, and there were no differences in the VAL or UPS temperatures between the heated and unheated conditions. DNS temperature was higher during the heated condition at all time periods (P< 0.01). Mean DNS temperatures for the heated and unheated condition were 30.2 ± 1.0°C and 27.9 ± 1.1 °C, respectively. We concluded that expired air temperatures near the pneumotachometer remain stable during extended steady-state exercise regardless of whether the pneumotachometer is heated or not.