Acclimation is one factor used to reduce heat stress. Time away from the warm environment may result in loss of acclimation. To reduce the risk of heat stress, a period of re-acclimation may be appropriate. PURPOSE: To establish the time for re-acclimation after periods of 2 and 4 weeks absence from the heat. METHODS: Ten healthy participants (4M, 6F) underwent an acclimation period of approximately 2 weeks by walking on a treadmill in a climatic chamber set at 50° C, 20% rh. The treadmill speed and grade was set to elicit a moderate metabolic rate of approximately 40% max VO2 as previously determined from a maximal treadmill test. Trials lasted 120 minutes unless one of the termination criteria was met. Semi-nude weight was recorded pre- and post-trial, and fluid intake was recorded. Heart rate (HR) and rectal temperature (Tre) were monitored continuously and recorded every 5 minutes during the trials. In addition, VO2 was taken at 30 minute intervals. A three day plateau in Tre was used as evidence of acclimation. After the acclimation periods, participants were randomly assigned to one of 2 re-acclimation groups and refrained from physical activity in the heat for a period equal to 1/3 (2 wks) and 2/3 (4 wks) complete loss of acclimation (6 wks) determined from previous research in our lab. Each group underwent the acclimation process until Tre reached a plateau for 3 days. A repeated measures design was used with Tre, HR, sweat rate and VO2 as the dependent variables. Trial (re-acclimation or re-acclimation) and re re-acclimation group were the independent variables. RESULTS: The average time for acclimation was 8.2+/-1.5 days. The average time for re-acclimation for the 2 wk and 4 wk groups was 6.2+/-0.8 and 6.6+/-2.0 respectively. There was no trial by re-acclimation interaction in our subjects. As expected, there were significant differences among the trials in end time, end HR and end Tre. End time was less and end HR greater for the beginning of acclimation and re-acclimation than for the end of acclimation and re-acclimation (end time = 88.5, 104.5, 120 and 120 minutes; end HR = 153.6, 157.8, 137.9, 129.5 bpm). End Tre was less for the end of reacclimation (Tre = 38.06°C) than for the beginning of acclimation (Tre = 38.63°C). CONCLUSION: In our participants, time of absence from the heat did not affect the time for re-acclimation or related physiological variables. As expected, participants reached acclimation in approximately 7 days in both acclimation and re-acclimation, yet time away from the heat did not influence re-acclimation time.