no presently available drug acts predominantly as a myofilament sensitizer in situ. We have investigated the effects and the mechanism of action of novel diazinone derivatives, EMD 54622, EMD 53998, and EMD 54650 (developed by E. Merck, Darmstadt), on guinea pig myocardial preparations. Forceand ATPase-pCa relations in skinned fibers show differing potencies of these agents on myofilament sensi...