The herbicidal 3,6-substituted 4-amino-1,2,4-triazin-5-one of the general formula ##STR1## wherein R.sup.1 and R.sup.2 have the meaning given in the description,is obtained in good yields by a new two-stage cyclization reaction if an .alpha.-ketocarboxylic acid amide of the general formulaR.sup.1 --CO--CO--NHR.sup.3 (II)is reacted with a hydrazidine of the general formula ##STR2## if appropriate in the presence of a diluent at temperatures between 0.degree. and 150.degree. C., whereby in the first process stage a condensation reaction takes place to give condensation products which still have open chains, and in the second process stage an elimination reaction with ring closure takes place to give the end product of formula (I).