This is a ARMY AEROMEDICAL RESEARCH LAB FORT RUCKER AL report procured by the Pentagon and made available for public release. It has been reproduced in the best form available to the Pentagon. It is not spiral-bound, but rather assembled with Velobinding in a soft, white linen cover. The Storming Media report number is A856504. The abstract provided by the Pentagon follows: The current study evaluated the feasibility of a 5-hour flight in MOPP4 with environmental conditions of l00 F and 50% relative humidity. Eight volunteer pilots participated in a within subject, repeated measures design. The test conditions included 70 F without nuclear, biological and chemical (NBC) gear (Cool Standard (CS)), 70 F with NBC gear (Cool MOPP (CM)), and 100 F with NBC gear and microclimate cooling (Hot MOPP (HM)). The test sessions consisted of an initial 20-minute warm up, two flight sorties and an intervening break (without NBC gear removal). Physiological parameters, flight performance, mood, and reaction time were measured during the test sessions. The peak mean core temperature did not exceed 99.8 F. Mean core temperature in the HM condition was about 0.4 F higher than CM and about 0.6 F higher than CS. The absolute increase was quite modest compared to the approved upper limit of l02.5 F. Mean percent dehydration was greatest for the HM condition despite the use of the cooling vest. However, the fluid deficit of approximately 300 cc per hour was a totally compensable rate. The results indicated that the cooling vest was effective in preventing heat stress during an extended rotary-wing mission in a hot, humid condition.