The simple and interactive effects of CO exposure and prior physical work on cognitive performance were evaluated in 16 subjects in two hot (WBGT = 30 degrees C) environments. Three levels of carboxyhemoglobin (COHb) (0, 7, 10%) and three workloads (rest, 35%, 60% VO2max) were crossed resulting in nine repeated measures conditions per subject. A bolus + ambient air maintenance technique was used to achieve the targeted COHb levels. Following administration of CO by bolus, subjects either exercised or rested for 50 min, then performed five cognitive tasks: Manikin spatial processing, Sternberg memory, Stroop word-color, visual search, and visual tracking, with and without a secondary mathematics task. The only cognitive impairment associated with an elevated COHb was seen in performance of the second of two sequentially presented Stroop test versions using the same stimuli but with competing instructions. Heat exposure per se had no significant effects on cognitive performance based on comparisons with other subjects who underwent the same protocol in a thermoneutral environment. Elevated COHb was associated with greater reporting of exertion and eye, ear, nose, and throat symptoms during heavy exercise concomitant with greater minute ventilation and heart rate. Except for the latter, these effects were not seen in thermoneutral conditions.