The effects of acetazolamide on renal and erythrocyte carbonic anhydrase were studied in 12 critically ill patients. In the first part of the investigation (n = 6) we examined the renal effects of increasing doses of acetazolamide. The maximal renal excretion of water and bicarbonate was achieved with acetazolamide 2.5–5 mg kg⁻¹x i.v. In the second part (n = 6), the associated respiratory effects of the effective renal dose of acetazolamide 5 mg kg⁻¹ were evaluated. We found a statistically significant 4% decrease in pulmonary carbon dioxide excretion in the 10-min sampling period immediately following the administration of acetazolamide, but thereafter carbon dioxide elimination proceeded at a normal rate. The observed carbon dioxide retention is clinically unimportant, and we recommend acetazolamide as an seffective means of eliminating surplus water and bicarbonate in the critically ill.