The thermal balance of 13 term infants was measured in a closed-circuit metabolism chamber. Each was studied naked, then with a gamgee-lined hat, and finally with a 'cummerbund' made of a similar material and of similar dimensions. At 27 degrees C the oxygen consumption of the 'hatted' babies was only 85% and the total heat loss 75% of the values measured with the infants naked. The cummerbund offered no detectable benefit. An additional 10 infants were studied while wearing a tubegauze hat at environmental temperatures of 28.5 (+/- 0.5) degrees C. This type of hat gave no measurable thermal protection. It is concluded that a substantial reduction of thermal stress in adverse environments can be achieved simply and clearly by adequately covering the vault of the skull.