## Abstract

A CO<latex>$_2$</latex> rebreathing test was used to determine the breathing pattern and the ventilatory response to CO<latex>$_2$</latex> in 15 Caucasians and 140 New Guineans (coastal and highland men and women, and male highlanders on the coast). The breathing pattern was analysed in terms of the slope and intercept (M and K) of the linear regression of ventilation on tidal volume: <latex>$V_e = M(V_t-K)$</latex>, and of the interpolated tidal volume at a ventilation of 30 l min<latex>$^{-1}$</latex> (V<latex>$_{t, 30}$</latex>). Each of these parameters bears a common relation to vital capacity throughout the groups studied. The CO<latex>$_2$</latex> response was analysed in terms of the slope and intercept (S and B) of the linear regression of ventilation on <latex>$P_{CO_2}:V_e = S(P_{CO_2} - B)$</latex>. B is lower in women than in men. S is a function of vital capacity, and this relation accounts for the difference in CO<latex>$_2$</latex> sensitivity between men and women, and for part of the difference between the resident highland and coastal groups; part is attributable to altitude-adaptation and disappears on migration. In all these respects, New Guineans resemble Caucasians, and the results demonstrate the importance of the size of the vital capacity in influencing the setting of the respiratory control mechanisms. In addition, there is a residual difference between the ethnic groups, with the New Guineans having the lower CO<latex>$_2$</latex> sensitivities and thus a greater tolerance of CO<latex>$_2$</latex> loads.