Bicarbonate was found to stimulate ATP breakdown by rabbit or cat ciliary body-iris homogenates. Maximum HCO3- stimulation of ATPase with Tris-Hepes buffer occured at pH 8.0. Acid pH and chloride ions in the media reduced the activity of the HCO3--stimulated ATPase. The Km for ATP was 0.55 mmolar and for HCO3-, 20 mmlar. HCO3- ATPase was not inhibited by acetazolamide added to in vitro. It is postulated that ATPase represents the linkage step of energy donor mechanism and active CT secretion in acid aqueous humors (human, cat.) or HCO3- secretion in alkaline aqueous humor (rabbit, guinea pig). Inhibition of Cl- or HCO3- secretion by acetazolamide results from decreased intracellular HCO3- levels which, in turn, reduces the stimulation of the HCO3- ATPase.