The electrochemical reduction of the herbicide Metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one) has been carried out at different pHs using a mercury pool cathode. The final product of these reductions depends both on the electrolysis potential and on the pH. All processes are irreversible and the reductions of the C = N and the N - NH2 (pH > 3) or N - NH3+ bond (pH < 3) are inferred. An aqueous solution of Metamitron is also degraded by sunlight leading to 4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one (deaminometamitron). This reaction can be monitored polarographically.