1. We have determined the ventilatory response to CO2 inhaled in 30% oxygen (balance nitrogen) in eight normal subjects (1) before and during 4 days of 80 mg of oral frusemide daily and (2) within 55–75 min of 80 mg of frusemide orally.

2. Over 4 days the drug decreased serum potassium concentrations, but increased end tidal (and arterial) Pco2 and serum bicarbonate, thus inducing a mild metabolic alkalosis with an appropriate but small shift in CO2 response to the right without a significant change in the slope of the response. The CO2 response was unaltered by oral frusemide 55–75 min earlier.

3. This slight rise in Pco2 during 4 days of frusemide therapy contrasts with the absence of rise in Pco2 after treatment with thiazide diuretics, as reported by others.

4. We discuss possible implications of these results for the selection of an appropriate diuretic in patients with CO2 retention at various phases of their illness.

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