The electrochemical behaviour of the herbicide 4-amino-3-methyl-6-phenyl-1,2,4 triazine-5(4H)-on (Metamitron) is studied in aqueous medium using voltammetric techniques, with the ionic strength adjusted to 0.1 mol/l in sodium perchlorate and using a Britton-Robinson buffer. Two reduction waves on the mercury drop electrode appear, at −0.49 V the first and the second folded at −0.95 V and −1.05 V. The system is identified as irreversible and fundamentally controlled by diffusion. Using differential pulse polarography the detection limit reached was 0.02 mg · l−1 for the first wave with an error of less than 2%. Thus a method is proposed for the determination of Metamitron in soil, with a detection limit of up to 0.02 μg/g.