This study examined the role of alterations in the chemoreflex control of breathing, acid-base balance and their interaction in postmenopausal ventilatory adaptations. A modified iso-oxic hyperoxic- and hypoxic-CO₂ rebreathing procedure was employed to evaluate central and peripheral chemoreflex drives to breathe, respectively, in 15 healthy postmenopausal (POST) and 20 premenopausal (PRE) women of similar age. Arterialized venous blood samples were collected at rest for the estimation of arterial PCO₂ (PaCO₂) and [H⁺]; plasma strong ion difference ([SID]) and total weak acid ([A_TOT]) concentrations; and serum progesterone ([P₄]) and 17-estradiol ([E₂]) concentrations. In POST vs. PRE: PaCO₂, [SID] and the central chemoreflex ventilatory recruitment threshold for PCO₂ (VRTCO₂) were higher, while [P₄] and [E₂] were lower (all p<0.05) with no significant change in central or peripheral chemoreflex sensitivity, peripheral chemoreflex VRTCO₂ and [A_TOT]. The acidifying effect of an increased PaCO₂ was offset by the alkalizing effect of an increased [SID], such that [H⁺] was preserved in POST vs. PRE. PaCO₂ correlated positively with the central chemoreflex VRTCO₂ (r=0.67, p<0.01), which in turn correlated positively with [SID] (r=0.53, p<0.01) within the pooled data. In conclusion, the relative alveolar hypoventilation and attendant arterial hypercapnia in healthy post- compared with premenopausal women could be explained, in part, by the interaction of (1) reduced central, but not peripheral, chemoreflex VRTCO₂, (2) increased [SID] and (3) reduced circulating female sex steroid hormone concentrations.