The validation of analytical procedures using added samples as reference materials is proposed. The carbetamide extracted with dichloromethane is monitored by reverse-phase high performance liquid chromatography with a retention time of some 2.7 min. while Carbaryl is used as an internal standard. The accuracy of the method is checked analysing water samples previously spiked with different amounts of analyte. A method to obtain the signal associated to a chromatographic blank is presented, to include as another precaution, within the calibration procedures. The method was applied to the determination of carbetamide at very low concentration levels (2.50-10.0 µg · L −1 ) in different types of natural water samples. The detection limit (DL) was 0.02 µg · L −1 . Around 100% recovery levels were customarily obtained in all cases. 355