A process for the preparation of a 4-methylpyrimidine of the general formula I ##STR1## in which R.sup.1 denotes C.sub.1 -C.sub.20 alkyl, C.sub.3 -C.sub.8 cycloalkyl, aryl, C.sub.7 -C.sub.12 phenalkyl, C.sub.7 -C.sub.12 alkylphenyl, NH.sub.2, NHCN, OH, and SH,in which a 1-aminovinyl methyl ketone of the general formula II ##STR2## in which R.sup.2 and R.sup.3 denote C.sub.1 -C.sub.20 alkyl, C.sub.3 -C.sub.8 cycloalkyl, aryl, C.sub.7 -C.sub.12 phenylalkyl, C.sub.7 -C.sub.12 alkylphenyl, C.sub.1 -C.sub.20 hydroxyalkyl or together denote a C.sub.2 -C.sub.7 alkylene chain optionally mono- to tetra-substituted by C.sub.1 -C.sub.4 alkyl and optionally interrupted by oxygen, nitrogen, or sulfur, is caused to react with a carboxamide or amidine or a salt thereof of the general formula III ##STR3## in which R.sup.1 has the aforementioned meanings and x stands for oxygen or NH, at temperatures ranging from 20.degree. to 200.degree. C. and pressures ranging from 0.01 to 50 bar.