The reaction between 4-(4-methylphenyl)but-3-en-2-one and aminoguanidine produced an unexpected product of formula C(12)H(15)N(3)O, consisting of a carboxamide moiety joined to a substituted pyrazoline ring at one of the N atoms. The pyrazoline ring adopts a flat-envelope conformation and the substituted phenyl ring is oriented almost perpendicular to the heterocycle. The carbonyl O atom has partial anionic character as a result of the transfer of pi density from the two adjacent sp(2) N atoms and is involved in an intermolecular hydrogen bond with the amide group.