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A group of men are studied who perform shift work at high altitude (4200 m) with five day work periods (shifts) at altitude interspersed with five days residence at sea level. Four shifts comprise a work cycle, with each cycle followed by a 40-day period at sea level. Shift workers are compared with a control group of sea level residents. Breath rate, minute volume of respiration, and alveolar ventilation increase at altitude for both shift workers and controls. Shift workers have a faster breath rate and lower tidal volume than do controls both at sea level and altitude. Heart rate significantly increases at altitude for controls and also for shift workers on the first day of their first shift. Pulse rate returns to sea level values after the first day in shift workers and does not increase during initial exposure during the third shift of a commuting cycle. Hematocrit and hemoglobin con­centration increase in the shift workers during a commuting cycle. The results suggest that shift workers attain partial acclimatization during a five- day shift and are able to carry over some of this acclimatization into suc­cessive shifts in a cycle. There is no indication of a carry over process from one cycle to another, however.