D ominant mutations in sarcomere protein genes cause hypertrophic cardiomyopathy, an inherited human disorder with increased ventricular wall thickness, myocyte hypertrophy, and disarray. To understand the early consequences of mutant sarcomere proteins, we have studied mice (designated αMHC^403/+) bearing an Arg403Gln missense mutation in the α cardiac myosin heavy chain. We demonstrate that Ca²⁺ is reduced in the sarcoplasmic reticulum of αMHC^403/+ mice, and levels of the sarcoplasmic reticulum Ca²⁺-binding protein calsequestrin are diminished in advance of changes in cardiac histology or morphology. Further evidence for dysregulation of sarcoplasmic reticulum Ca²⁺ in these animals is seen in their decreased expression of the ryanodine receptor Ca²⁺-release channel and its associated membrane proteins and in an increase in ryanodine receptor phosphorylation. Early administration of the L-type Ca²⁺ channel inhibitor diltiazem restores normal levels of these sarcoplasmic reticular proteins and prevents the development of pathology in αMHC^403/+ mice. We conclude that disruption of sarcoplasmic reticulum Ca²⁺ homeostasis is an important early event in the pathogenesis of this disorder and suggest that the use of Ca²⁺ channel blockers in advance of established clinical disease could prevent hypertrophic cardiomyopathy caused by sarcomere protein gene mutations.