We investigated the inhibitory effects of increased salt concentrations on maximal calcium-activated force (Fmax) of rabbit cardiac papillary muscle bundles skinned with Triton X-100. While other studies have reported a lack of ion-specific effects on Fmax of cardiac muscle, we clearly demonstrated the presence of such effects when a wider variety of salts was investigated. In addition, like skeletal muscle, cardiac muscle was found to be sensitive to ionic strength and not to ionic equivalence. In support of our hypothesis that the ion-specific effects are due to protein destabilization, we found that a protein stabilizer (trimethylamine N-oxide, TMAO) completely abolished the ion-specific effects on Fmax. The ion-specific effect is probably due to binding of ions to the contractile proteins. The general ionic effect is most likely due to electrostatic shielding that remains in the presence of TMAO. Neither 300 mM sucrose nor TMAO significantly altered Fmax at physiological ionic strength indicating that the ion-specific depression of Fmax was not due to a colligative/osmotic effect. Furthermore, adding sucrose to solutions with a supraphysiological ionic strength caused a further decrease in Fmax indicating that certain osmolytes can alter Fmax if the contractile proteins are initially destabilized.