In the preparation of 4-amino-6-tert.-butyl-3-methylthio-1,2,4-triazin-5-(4H)-one of the formula ##STR1## wherein pivaloyl cyanide of the formula(CH.sub.3).sub.3 C--CO--CNis reacted to form an intermediate, the intermediate is condensed with thiocarbohydrazide of the formulaNH.sub.2 --NH--CS--NH--NH.sub.2to form 4-amino-6-tert.-butyl-3-mercapto-1,2,4-triazin-5-(4H)-one of the formula ##STR2## and the 4-amino-6-tert.-butyl-3-mercapto-1,2,4-triazin-5-(4H)-one is alkylated to replace the H atom on the sulphur by --C.sub.1-4 --alkyl, the improvement which comprises reacting the pivaloyl cyanide with a carboxylic acid anhydride of the formulaR--CO--O--CO--Rin which R is an optionally substituted aliphatic radical with up to 8 carbon atoms or an optionally substituted phenyl radical, in the presence of a strong acid at a temperature between about -50.degree. and +150.degree. C., and then directly reacting the reaction mixture thus obtained with the thiocarbahydrazide. Advantageously R is CH.sub.3, the strong acid is concentrated sulphuric acid, the pivaloyl cyanide is effected at a temperature between about 0.degree. and 100.degree. C., the condensation with thiocarbohydrazide is effected, in the presence of a mineral acid, in an aqueous or aqueous-alcoholic medium, and the alkylation is effected using methyl iodide or methyl bromide in the presence of sodium hydroxide at about 0.degree. to 50.degree. C.