A study has been conducted on 20 sojourners (SJs) at a high altitude (HA) of 3500 m to evaluate the changes in thermoregulation efficiency (ThE) during acclimatization for 3 weeks, by observing the rate and pattern of rewarming of palm skin temperature (Tsk) after dipping the hand in water of 10 +/- 1 degrees C for 2 min (Tromp's water bath test). The ThE of the SJs was compared to that of 10 acclimatized lowlanders (ALs) staying at the same altitude for about 1 year and 10 high-altitude natives (HANs). In SJ, at sea level (SL), the effect of seasonal variation and ambient temperature on ThE was also studied. The initial Tsk of the palm (29.9 degrees C) was lower at altitude as compared to SL value (32.2 degrees C) in SJs inspite of similar ambient temperature (21 +/- 1 degrees C). There was a prolongation of rewarming time in all the subjects during stay at altitude. The rewarming curve showed a shift towards the right, indicating slower rewarming. The rewarming response of ALs and SJs was comparable at altitude. In HANs, the rewarming at altitude was faster and resembled those of SJs at SL. The rewarming improved in the season following cold months, and at higher ambient temperature in the same season, on the plains. It may be concluded that thermoregulation efficiency of man deteriorates at high altitude, and the peripheral vascular responses to local cold stress do not reach the level attained by natives even after 1 year of acclimatization. General cold acclimatization improves the rewarming response.