It is generally recognized that ventricular myosin regulatory light chains (RLC) are ∼40% phosphorylated under basal conditions, and there is little change in RLC phosphorylation with agonist stimulation of myocardium or altered stimulation frequency. To establish the functional consequences of basal RLC phosphorylation in the heart, we measured mechanical properties of rat skinned trabeculae in which ∼7% or ∼58% of total RLC was phosphorylated. The protocol for achieving ∼7% phosphorylation of RLC involved isolating trabeculae in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC from its baseline level. Subsequent phosphorylation to ∼58% of total was achieved by incubating BDM-treated trabeculae in solution containing smooth muscle myosin light chain kinase, calmodulin, and Ca²⁺ (i.e., MLCK treatment). After MLCK treatment, Ca²⁺ sensitivity of force increased by 0.06 pCa units and maximum force increased by 5%. The rate constant of force development (k_tr) increased as a function of Ca²⁺ concentration in the range between pCa 5.8 and pCa 4.5. When expressed versus pCa, the activation dependence of k_tr appeared to be unaffected by MLCK treatment; however, when activation was expressed in terms of isometric force-generating capability (as a fraction of maximum), MLCK treatment slowed k_tr at submaximal activations. These results suggest that basal phosphorylation of RLC plays a role in setting the kinetics of force development and Ca²⁺ sensitivity of force in cardiac muscle. Our results also argue that changes in RLC phosphorylation in the range examined here influence actin-myosin interaction kinetics differently in heart muscle than was previously reported for skeletal muscle.