Plasma potassium (K+) concentrations were elevated to 7.3 mEq/l (mean) and correlated with arterial blood hydrogen ion (H+) concentrations which averaged 72 nmoles/liter, (pH 7.14), r = 0.541 during acute severe respiratory acidosis in 7 patients. During severe acidosis erythrocyte volumes were 10% above values after its relief. Equilibration of human blood with 20-30% CO2 to produce equivalent acidosis increased erythrocyte volumes similarly but raised plasma K levels only slightly. Brown and Goott showed in dogs breathing 30% CO2 that despite a greater increase in intracellular hydrogen ion concentration (H+)i than in extracellular levels (H+)e during respiratory acidosis the ratio of (H+)i to (H+)e decreased. Concomit-antly, plasma K increased and was highest in venous blood from muscles. An analogous decrease in the (H+)i to (H+)e ratio probably occurs in patients with respiratory acidosis. Their plasma K levels increased acutely in proportion to the severity of acidosis and remained elevated but they decreased promptly when hypercapnia was relieved.