Procaine hydrolysis in vitro has been studied in whole blood, plasma, and washed erythrocytes. Esterase activity was higher in whole blood than in either diluted plasma or resuspended erythrocytes. Eserine and echothiophate specifically inhibited plasma procaine esterase activity, while acetazolamide blocked hydrolysis of procaine by washed erythrocytes. Kinetic studies in whole blood also identified 2 different enzymes. Procaine esterase activity associated with red blood cells was not impaired in patients with renal failure or hepatic cirrhosis, but procaine half-life (t 1/2) in whole blood of normal subjects was longer after 250 mg acetazolamide.