Abstract A rapid, precise and low cost spectrophotometric method is proposed for the determination of methyldopa and dopamine in pharmaceutical formulations. The crude extract of sweet potato root ( Ipomoea batatas (L.) Lam.) was used as an enzymatic source of polyphenol oxidase (PPO; EC.184.108.40.206). This enzyme catalyses the oxidation of catecholamines to the corresponding methyldopaquinone and dopaminequinone. Those compounds are converted by a rapid spontaneous auto-oxidation to methyldopachrome and dopaminechrome which have a strong absorption at 480 or 470 nm, respectively. The calibration graphs are linear from 2.0×10 −4 to 6.0×10 −3 M. The results obtained by the proposed enzymatic method are in close agreement with those obtained using a Pharmacopoeia procedure and also with the label values. The detection limit (three times the signal blank/slope) was 3.4×10 −5 and 3.0×10 −5 M for methyldopa and dopamine, respectively, the recovery of methyldopa and dopamine from three samples ranged from 97.5 to 102.9% of the added amount.