Preparation of tetrahydropyrimidines

3 - Alkyl - 2 - substituted - 3,4,5,6 - tetrahydropyrimidines in which the 2-substituent is an alkyl group or aryl hydrocarbon group, optionally substituted by halogen, are prepared by heating an acylated-N-alkyl-1,3-propane diamine in the presence of a catalyst consisting of nickel, copper, cobalt, manganese, platinum or palladium or salts thereof and in the presence of a strong base at a temperature in the range 180-200 DEG C. The acylated N-alkyl-1,3-propane-diamines may be prepared by reacting an N-alkyl, N-b -cyanoethylamine, having from 8 to 22 carbon atoms, with an acylating agent derived from a carboxylic acid and hydrogenating the product over a nickel catalyst under superatmospheric pressure and in the presence of a base.