A series of 30 6-chloro-2,4-diaminopyrimidines was synthesized and tested in vitro as inhibitors of [3H]spiroperidol binding. The affinity for the dopamine receptor was shown to be related to the 6-chloro-4-(N-methyl-piperazine)pyrimidine structure bearing a NH2 or NHR1 group as a substituent in position 2, provided that R1 was not an alpha branched alkyl group. The nature of the substituent in position 5 is also of importance for the affinity; 2-(benzylamino)-6-chloro-4-(N-methylpiperazino)-5-(methylthio)pyrimidine (22) is the most active member of the series. Molecular structures of three compounds were analyzed by X-ray diffraction and PCILO computation.