After a 30-s breath-hold (BH), expired temperature (TE) does not reach core temperature. One explanation is that the gas in the airways is not in thermal equilibrium with the airway walls. This possibility was eliminated by comparing TE in six subjects breathing either helium-oxygen or air after a BH. Another possibility is that the airway walls and surrounding tissues have sufficient thermal inertia to slow down thermal equilibrium during BH. This was checked by measuring oral and upper esophageal temperatures after cooling or heating the airways. It took more than 2 min for these temperatures to recover their steady-state value. Six subjects were requested to perform a long apnea after hyperventilating for 1 min and then taking a single breath of 100% oxygen. TE was still lower than core temperature after a 1-min BH, and there was no difference after a 2-min BH. The difference between expired and core temperatures during BH thus appears to be due to the thermal inertia of the airways and their surrounding tissues.