A carbon fiber ultramicroelectrode was used for the electroanalytical determination of epinephrine in biological fluids (urine) by cyclic voltammetry. The ultramicroelectrode was subjected to an electrochemical pretreatment in order to improve the epinephrine adsorption on the electrode surface. With this preconcetration step, detection limits of 7.8 ng ml(-1) and determination limits of 27.6 ng ml(-1) can be reached. The method was contrasted with native fluorescence determination-similar results were obtained.