At two altitudes (3,600 and 5,200 m), resident native women (A) showed higher PAco2 and Paco2, lower PAO2 and PaO2, as well as lower arterial pH, than native males (C) of similar age. Resting minute ventilation (VE/m2), tidal volume (VT) and respiratory frequency (f) were similar. The differences in the alveolar-arterial gas tensions in A stem from larger dead space (VDS), VDS/VT and reduced alveolar ventilation (VA). Newcomer females (B) at both altitudes differed from new- comer males (D) in regard to larger resting VA, f and smaller VT. No differences were demonstrated in size of VDS, VDS/VT or VO2/BSA; but PAco2 and PacO2 were lower, PAO2 and Pa higher and VE/VO2 and VA/VO2 (ventilatory equivalents) above those for resting newcomer males. A contrasted with resting B at 3,600 m (12,200 ft), showing higher PAco2 and Paco2, lower Pao2 and arterial pH, reduction in VA and larger VDS/VT. Both C and D at 3,600 and 5,200 m during steady-state 60-W (367 kg/m/min) exercise showed lower PAco2 and Paco2, higher arterial pH, smaller VDS and higher VE/VO2 than exercising A. The latter differed from C exercising at the same work load in that VE/m2 and VA were reduced, PAco2 and Paco2 were higher, Pa-ao2 gradients were wider, pH was lower and VDS as well as VDS/VT were larger. Contrasted with B during steady-state 60-W exercise at both altitudes, D showed consistently smaller VDS and larger VA. While both C and A had higher resting f than D, f of newcomer females at La Paz was higher than that of A. Control of ventilation by relating VE to Pao2 and Paco2 at rest and exercise at both altitudes showed marked attenuation of hypoxic and hypercapnic drives, particularly in the native resident female, suggesting diminished peripheral chemoreceptor function.