New substituted 4-aryl-2,5-dioxo-8-phenylpyrido[2,3-d]pyridazines 4a-f have been prepared in one step from the corresponding arylidene substituted Meldrum's acid (1) and 5-amino-6-phenyl-3(2H)-pyridazinone (2) in good yields. Semiempirical theoretical calculations (AM1) reveal two favoured conformations (A and B) for compounds 4a-f, with a screw boat conformation in the pyridone system and a planar pyridazinone ring. X-Ray crystallographic analysis shows that in the solid state, conformation A bearing the phenyl ring in a pseudoaxial position is the most stable. Compounds 4a-f fulfil, from the structural point of view, all the requirements needed for exhibiting cardiotonic effects. (C) 2000 Elsevier Science Ltd. All rights reserved.