Compared with the induction of heat acclimation (HA), studies investigating the decay and re-induction of HA (RA) are relatively sparse and have yielded conflicting results (Pandolf, 1998). Consequently, the aim of this study was to quantify HA decay and the number of days required to attain RA following two periods without exposure to heat stress. Following local ethics approval, 16 semi-nude men (mean ± 1 S.D.: age, 28.1 ± 7.2 y; body mass, 74.2 ± 10.6 kg; peak oxygen uptake (VO₂peak), 51.9 ± 5.2 ml min^-1 kg^-1) were acclimated to dry-heat by undertaking an exercise protocol in a hot chamber (dry-bulb temperature, 46.1 ± 0.1 °C; relative humidity, 17.9 ± 0.1 %) on 10 consecutive days (HA1–10) in winter UK. Thereafter, the subjects were divided into two groups and re-acclimated to heat for up to 7 days (RA1–7) after 12 (Group 1) and 26 days (Group 2) without exposure to heat stress. The exercise protocol consisted of 60 minutes of treadmill walking at a fixed speed/incline (45 ± 4 %VO₂peak), followed by a 10-minute rest period and 40 minutes of treadmill walking/running, the intensity of which was increased across HA to maintain rectal temperature (T_re) at ~38.5 °C. T_re, mean skin temperature (T_sk), heart rate (f_c) and rate of water loss (m_wl) were recorded. At 60 minutes, T_re, T_sk, f_c and m_wl did not change after HA7, and HA was taken as the mean of HA8–10. 60-minute heat strain data at HA8–10 and RA1, and the day at which RA was attained are given in Table 1. For both groups there was no decay in T_re, and for all measured variables, RA was attained by RA2 and RA4 in Groups 1 and 2, respectively. These results are of practical significance to all individuals working/competing in hot environments, and suggest that once adaptation to heat has been attained, the time that individuals may spend in cooler conditions before returning to a hot environment could be as long as one month, without the need for extensive re-adaptation to heat.