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There is a paucity of evidence to support the hypothesis that there is no significant difference (NSD) between the rate of acclimatization gain and loss of young and old men as they work in a hot environment. Ten young men (Group 1 (x age 24.8 years)) and 5 older men (Group 2 (x age 64.4 years)) volunteered to complete a 39 day exercise regimen in an environmental chamber. Days 1-4 were the Baseline Phase (BL) (68-72 F{dollar}\\sp\\circ{dollar} dry bulb (DB), relative humidity (RH) 40%). Days 5-18 were the Acclimatization Phase (AC) (DB = 100 F{dollar}\\sp\\circ{dollar}, RH = 40%). Days 19-39 were the Acclimatization Loss Phase (AL). During AL, the BL environment was maintained for 6 days followed by a retest on the seventh day when the environment was heated the same as during AC. AL continued for 3 weeks. The following variables were monitored to assess the physiologic responses to treatment: surface area, percent body fat, maximum oxygen consumption (Max VO{dollar}\\sb2{dollar}), hematocrit, hemoglobin, rate pressure product, cardiac output, workload (WL), perceived exertion, sweat rate, and skin and rectal temperature. Participants exercised for 50 continuous minutes each day at 40% of their Max VO{dollar}\\sb2{dollar}. Group 1 experienced acclimatization on Day 18 by achieving a WL of 88% of BL. Group 2 achieved only 64% of the BL WL by Day 18 and did not acclimatize. Following 1 week of AL, WL for Group 2 was significantly lower than that of Group 1 (p = 0.023). Following 2 and 3 weeks of AL there were NSD between groups in achieved WL nor in rate of acclimatization loss. It is recommended that older men acclimatize to work in heat by maintaining proper hydration, alternating light and heavy days, and using heart rate as an indication of WL.