The optical resolution of 6-(4-aminophenyl)-5-methyl-2,3,4,5-tetrahydropyridazin-3-one (I) with L-tartaric acid in isopropanol or D-tartaric acid in ethyl acetate gives the (R)-enantiomer (II), which is then condensed with malonodinitrile (III) by means of NaNO2 and HCl in cool water.