Biochar is a carbonaceous material that exhibits rich surface functional groups, which makes its use as a modifier of electrodes a very attractive alternative, especially the preconcentration of species of interest, organic or inorganic. This capability can be enhanced by activation of the material that increases the amount of functional groups, hence the ability to preconcentrate more species. In this study we used a carbon paste electrode modified with biochar activated with nitric add for spontaneous preconcentration of Methyl Parathion and for further quantitative determination in drinking water. The electrode showed good sensitivity and limits of detection of MP, 760 mu A L mmol(-1), 39.0 nmol L-1, respectively. Good accuracy of the method to determine MP in tap water samples was observed with recoveries values varying between 102 and 110% for three different concentration levels.