Abstract We have found previously that two 5-acetyl-4-methylpyrimidines could be transformed by aminoguanidine hydrochloride into 4-acetyl-1-amidino-3-methylpyrazolyl amidinohydrazone dihydrochloride. This reaction beside the fact that it led to a molecule closely related to the antitumor drug MGBG pointed out to a new possibility of ring contraction of pyrimidines into pyrazoles. We attempted therefore to study the possibility of ...