Square-wave adsorptive-stripping voltammetry technique has been used to develop a method for the determination of fenthion in olive oil. Due to the fact that fenthion does not give any electrochemical signal at mercury electrode, the method has been based on a previous oxidation of fenthion to its metabolite, fenthion-sulfoxide, by using KMnO(4). The metabolite gives rise to a peak due to an adsorptive-reductive process at -0.786 V. Fenthion is isolated from olive oil by carrying out a solid-liquid extraction procedure using silica cartridge, followed by a liquid-liquid partitioning with acetonitrile. The detection limit in olive oil is 78.8 ng g(-1) and recoveries for four levels of fortification are ranged from 85% to 109%. On the other hand, it has been developed a method for the simultaneous determination of fenthion and its metabolite fenthion-sulfoxide, in river water. Pesticides are isolated from water by carrying out a liquid-liquid partitioning with trichloromethane. The detection limits are 0.41 ng g(-1) and 0.44 ng g(-1), for fenthion and fenthion-sulfoxide, respectively. Recoveries for three levels of fortification are ranged from 96% to 103% for fenthion and 94% to 104% for fenthion-sulfoxide.