1. An analysis was made of the relationship which exists between arterial [H+], Pco2 and [HCO3−] in twenty-five patients with stable metabolic acidosis and alkalosis and in three normal subjects.
2. Contrary to previous reports, the relationship between Pco2 and [H+] was nonlinear and could best be described in terms of a rectangular hyperbola (Pco2 = 962/([H+]-12).
3. The relationship between Pco2 and [HCO3−] was curvilinear and best described by the quadratic function 23.8 (Pco2)2−12Pco2[HCO3−] −962 [HCO3−] = 0.
4. The small acute changes in [H+],−[HCO3−] and Pco2 produced by infusion of the weak organic acid 5,5−dimethyl 2,4−oxazolidinedione (DMO) could be predicted from the curvilinear regression.