The effect of carbonic anhydrase (CA) inhibition with acetazolamide (ACZ, 10 mg kg-1 I.V.) on the peripheral and central chemosensitivity and breathing pattern was investigated in four women and three men aged 25 ± 3 years using a modified version of Read's rebreathing technique. Subjects were exposed to dynamic increases in CO2> in hypoxic and hyperoxic backgrounds during control conditions and following acute CA inhibition. All manoeuvres were repeated twice and averaged for data analysis. The central chemoreflex sensitivities, estimated from the slopes of the ventilatory response to CO2 during hyperoxic rebreathing, increased following acute CA inhibition (control vs. ACZ treatment: 1.87 ± 0.66 vs. 4.07 ± 1.03 l min-1 (mmHg CO2)-1, P < 0.05). The increased slope was reflected by an increase in the rate of rise of tidal volume and breathing frequency. Furthermore with ACZ, there was a left-ward shift of the ventilation vs. end-tidal PCO2 curve during hyperoxic hypercapnia but not hypoxic hypercapnia. The peripheral chemoreflex sensitivity was isolated by subtracting the hyperoxic slope (central only) from the hypoxic slope (central and peripheral). Following ACZ administration, the peripheral chemosensitivity was blunted (control vs. ACZ treatment: 3.66 ± 0.92 vs. 1.33 ± 0.46 l min-1 (mmHg CO2)-1, P < 0.05). In conclusion, acute CA inhibition enhanced the central chemosensitivity to CO2 but diminished the peripheral chemosensitivity.