Inherited restrictive cardiomyopathy (RCM) is a debilitating disease characterized by a stiff heart with impaired ventricular relaxation. Mutations in cardiac troponin I (cTnI) were identified as causal for RCM. Acute genetic engineering of adult cardiac myocytes was used to identify primary structure/function effects of mutant cTnI. Studies focused on R193H cTnI owing to the poor prognosis of this allele. Compared with wild-type cTnI, R193H mutant cTnI more effectively incorporated into the sarcomere, where it exerted dose-dependent effects on basal and dynamic contractile function. Under loaded conditions, permeabilized myocyte Ca(2+) sensitivity of tension was increased, whereas the passive tension-extension relationship was not altered by R193H cTnI. Normal rod-shaped myocyte morphology acutely transitioned to a "short-squat" phenotype in concert with progressive stoichiometric incorporation of R193H in the absence of altered diastolic Ca(2+). The specific myosin inhibitor blebbistatin fully blocked this transition. Heightened Ca(2+) buffering by the R193H myofilaments, and not alterations in Ca(2+) handling by the sarcoplasmic reticulum, slowed the decay rate of the Ca(2+) transient. Incomplete mechanical relaxation conferred by R193H was exacerbated at increasing pacing frequencies independent of elevated diastolic Ca(2+). R193H cTnI-dependent mechanical tone caused acute remodeling to a quasicontracted state not elicited by other Ca(2+)-sensitizing proteins and is a direct correlate of the stiff heart characteristic of RCM in vivo. These results point toward targets downstream of Ca(2+) handling, notably thin filament regulation and actin-myosin interaction, in designing therapeutic strategies to redress the primary cell morphological and mechanical underpinnings of RCM.