The relationship between acid base parameters and serum potassium concentration was studied in controls and in patients during acute ammonium chloride-induced metabolic acidosis. Serum potassium was best correlated with serum bicarbonate during control (r = -0.323; p less than 0.01) and acidosis (r = -0.437; p less than 0.001). The slopes and intercepts were similar in both instances and the combined correlation was highly significant (r = -0.493; p less than 0.001). Examination of the 95% joint confidence region revealed that during acidosis serum potassium was rarely above 5.0 mEq/l when serum bicarbonate was greater than 16 mEq/l. It is probably not sound clinical practice to ascribe hyperkalemia to acute mild metabolic acidosis.