The method of the quantitative determination of procaine that has been adopted here is the colorimetric one which modified Brodie's; that is, in place of N (1-naphthyl) ethylendiamine in Brodie's method, more excellent Tsuda's reagent, 1-(ß-diethyl amino ethylamino) naphthalene dihydrochloride, has been used in ours, and buffer of boric acid (pH 9.0) has been used to separate procaine chemically from p-amino benzoic acid, hydrolyzed product of procaine produced by procainc-esterase in our method as well as in Brodie's. The procaine-esterase in 1 ml. of fresh human serum has the activity to he capable of hydrolyzing almost all the procaine hydrochloride 0.01 mg into p-amino benzoic acid and diethylamino ethanol in 4 to 5 minutes at shade temperature (10°C). The older the serum is in an ice stockroom, the more this activity of procaine-esterase in serum decreases. Procaine-esterase is scarcely existing in blood-corpuscles and extracellular fluid, such as fluid in bulla by burns or pleural exudation. Futhermore it is said that the narcotic action of diethylamino ethanol produced by the hydrolysis of procaine is weaker than that of procaine. Therefore it is more suitable for procaine to be used as local anesthesia than intravenous one.