An organic phase change material (PCM) possesses the ability to absorb and release large quantity of latent heat during a phase change process over a certain temperature range. This paper presents results related to thermo-physiological efficiency of special underwear with organic PCM integrated in textile through microencapsulation process. The efficiency of PCM underwear was tested through physiological examinations in simulated high-temperature conditions, where test-subjects were voluntarily exposed to heat stress tests wearing NBC protective suit with PCM underwear (option "THERM") and without it (option "NoTHERM"). It can be concluded that wearing a PCM textile clothes under NBC protective suit, during physical activity in high-tempearture conditions, reduces sweating and alleviates heat stress manifested by increased core and skin temperature and heart rate values.