Carbonic anhydrase activity in the extracellular fluid of lower vertebrates is considered to be minimal, either because of the absence of carbonic anhydrase or because of the presence of naturally occurring inhibitors. The presence of carbonic anhydrase activity and circulating inhibitors was measured in plasma and subcellular fractions of gill tissue in elasmobranchs and teleosts. Plasma carbonic anhydrase activity was confirmed in the former but in extremely low amounts, especially compared with activity in red cells. The activity was correlated with plasma iron concentration and red cell hemolysis, which suggests that it is a byproduct of endogenous hemolysis during red cell turnover. A subcellular fraction of dogfish gills rich in microsomes contained significantly higher carbonic anhydrase activity than previously found in teleosts, making elasmobranchs the only aquatic lower vertebrates to possess putative basolateral membrane-associated carbonic anhydrase in the gill vasculature. It is suggested that branchial membrane-associated carbonic anhydrase is correlated more with a pH and/or CO2-sensitive ventilatory drive than with the maintenance of resting CO2 excretion. The occurrence and effectiveness of plasma carbonic anhydrase inhibitors were highly species-specific, with the salmonids having the most potent inhibitor. Cross-reactivity of inhibitor to red cell carbonic anhydrase appeared to be related to phylogenetic proximity. Selection for the presence of carbonic anhydrase inhibitors in fish plasma appears to be the result of multiple physiological pressures, including preservation of red cell intracellular pH, ventilatory control, and red cell fragility.