A simple, lightweight microclimate cooling system may be required for soldiers conducting cold weather operations. In the future, many of the soldier's physiological systems (eg, heart rate) will be monitored in the field, and the data will be transmitted to remote sites. A technical system that allows for the monitoring of core temperature in the field is required. Although some systems are available to perform this function, most do not give robust or consistent recording and are not field hardened. Because most troops must be prepared for 24-hour deployment worldwide, they will not be acclimatized to either hot or cold temperatures (For a thorough review of human cold acclimatization interested readers may consult Young's chapter in the Handbook of Physiology.) s Constant exercises in the cold are required for maintaining combat effectiveness for cold weather operations. Although more "hot spots" are thought to be in hot climates and hyperthermia may be considered a greater