The effects of metabolic acidosis upon the pattern of apnea during sleep were assessed in ten sleep apnea patients. Four had pure obstructive apnea, two pure central apnea, and four had mixed apnea. Acidosis was induced with acetazolamide. Acid-base shifts had little effect in pure obstructive and pure central apnea, but had a significant effect in mixed apnea. In two of the mixed apneic patients, metabolic acidosis converted predominantly central apnea into nearly pure obstructive apnea, prolonging apneic periods and worsening hypoxemia. A suggested explanation for this is the greater stimulating effect of acidosis upon the lower bellows muscles than upon the muscles which act to maintain patency of the upper airways. The observation that some patients with mixed sleep apnea appear to have central apnea while relatively alkalotic and obstructive apnea while acidotic emphasizes the need for more careful and detailed characterization of apneic disorders with respect to their responses to body states and therapeutic agents.