The aim of this study was to analyse the acclimation of male Europeans during a forty-one day stay in the hot and humid climate of Thailand. We also tried to examine the phases of acclimation which would eventually be used by trainers in the elaboration of the schedule of athletes' preparation to participate in competitions in a tropical climate. Twelve Polish male subjects ageing 21-38 years participated in these examinations. In Poland and Thailand the cycloergometric exercise test with the load of 53% of VO2max was performed until a 1.2 degrees C (delta Tre) increase in rectal temperature was reached. The exercise test was executed in the same environmental conditions (i.e. 30 +/- 1 degrees C and 70 +/- 3% of relative humidity). The duration of this exercise test (DE) was used as a criterion for the efficiency of thermoregulatory functions. During acclimation, three peaks of greater exercise thermoregulatory efficiency have been found, i.e. on the 4th-5th, 11th-12th and 29th-30th days of stay. These findings are particularly important for professional athletes who wish to prepare themselves for competitions held in hot and humid climates.