In studies of 12 volunteer subjects, VC and ERV significantly decreased during a 3-d sojourn at 4,300 m while VE, VT, TLC, FRC and RV were elevated. Acid-base parameters showed typical changes associated with translocation to high altitude. Thus, PaO2, PaCO2 and SaO2 were immediately reduced upon translocation to 4,300 m, while the compensatory reduction in arterial HCO3- concentration was delayed temporarily by 24 h; pH, however, remained essentially unchanged throughout the sojourn. Upon relocation to 1,600 m, there was a gradual return to VC, ERV, TLC and RV to prealtitude values. FRC, on the other hand, remained elevated as did VE and VT through the third day of relocation to 1,600 m. PaCO2 and arterial HCO3- concentration showed a slight delay in returning to prealtitude values upon relocation, while the remainder of the acid-base measurements returned to prealtitude values within 24 h. The conclusion drawn from these results indicates a physiological adjustment period as long as 3 d may be required for individuals returning from a 72-h sojourn to high altitude (4,300 m).