1. At present, CO2 is considered to be the most important factor in regulating cerebral blood flow by modification of the interstitial fluid and extracellular pH, but the mechanism by which hypercapnia produces vasodilation is still controversial. In the present paper we investigated the effect of hypercapnia on carbonic anhydrase (CA) activity. We also studied the combined effects of CO2 with either indomethacin or an L-arginine analogue on CA activity. 2. Nine groups of 12 rabbits each were established. Groups 1-4 were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 20, 60, 120 and 180 min. Group 5 rabbits received 15 mg/kg bodyweight, i.v., indomethacin and, after 1 h, were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h. Group 6 animals were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h and then received indomethacin. Group 7 rabbits received 100 mg/kg bodyweight, i.v., NG-monomethyl-L-arginine (L-NMMA) and, after 1 h, were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h. Group 8 rabbits were ventilated for 2 h with a mixture of 10% CO2, 21% O2 and 69% N2 and were then administered L-NMMA. Group 9 rabbits received L-NMMA treatment concomitant with ventilation for 2 h with a mixture of 10% CO2, 21% O2 and 69% N2. In all groups, the erythrocyte CA activity was measured, as well as PaCO2 before and after ventilation or treatment. 3. The present study shows that CO2 reduces CA I activity down to complete inhibition and antagonizes the activating effects of indomethacin and L-NMMA on this isozyme. Our data prove that nitric oxide- and prostaglandin-induced CA I inhibition is involved in the vasodilation produced by hypercapnia. These results suggest that, due to subsequent pH changes, CA I is directly implicated in the modulation of vascular processes in the organism.