An electrochemical method has been described for the voltammetric oxidation and determination of methyldopa (MD) at a carbon paste electrode (CPE) modified with 5-amino-2'-ethyl-biphenyl-2-ol (5AEB) and carbon nanotubes (CNTs). The results indicated that the voltammetric response of MD was improved distinctly at the surface of modified electrode and the oxidation of MD at the surface of modified electrode occurs at a potential of about 220 mV less positive than that of an unmodified CPE. The anodic peak was characterized and the process was diffusion-controlled. The current measured by square wave voltammetry (SWV) presented a good linear property as a function of the concentration of MD in the range of 0.1-210.0 mu M, with a detection limit of 48.0 nM for MD. Also, this modified electrode was used for simultaneous determination of MD and hydrochlorothiazide (HCT) for the first time. Finally, the proposed method was successfully applied to MD and HCT determination in pharmaceutical samples and urine as real samples. (C) 2013 Elsevier B.V. All rights reserved.