A series of analogues of the cyclic AMP phosphodiesterase (PDE) inhibitor N-cyclohexyl-N-methyl-4-[(1,2,3,5-tetrahydro- 2-oxoimidazo[2,1-b]quinazolin-7-yl)oxy]butyramide (RS-82856, 1) was prepared by systematic variation of the side-chain substituent, length, position, connecting atom, and the parent heterocycle itself. The compounds were evaluated as inhibitors of cyclic AMP phosphodiesterase from both human platelets and rat or dog heart tissue and as inhibitors of ADP-induced platelet aggregation. Structure-activity correlations for the analogue series revealed significant limitations on the steric bulk of substituents on the 1,2,3,5-tetrahydroimidazo[2,1-b]quinazolin-2-one heterocycle and the position and length of the side chain. As inhibitors of cyclic AMP phosphodiesterase (PDE), potency steadily increased with increasingly lipophilic side chains. In platelet aggregation inhibition studies, however, a maximum in activity was reached with 1, while more lipophilic compounds were significantly less active. Major changes in the heterocycle itself, represented by isomeric and other carbonyl variations, also decreased activity. The molecular features defined by this series of analogues of 1 correlate to a high degree with current understanding of the chemical and topographical requirements of the active site of the FIII (type IV) form of cyclic AMP PDE. Selective inhibition of this enzyme has been proposed as the principal component of the positive inotropic action of a number of cardiotonic agents.