Laboratory rats were exposed to experimental concentrations of carbon monoxide in air at ambient temperature, to elevated temperature atmospheres from 40° C to 60° C, and to selected CO concentrations in 40-60° C whole-body environments. Incapacitating potency was evaluated by measuring time-to-incapacitation as a funktion of CO concentration and/or temperature. Incapacitation occurred earlier when CO inhalten was combined with elevated temperature than when the same parameters were applied individually; a fractionally additive effect was noted. An empirical equation was derived for predicting time-to-incapacitation from CO concentration and temperature data.