Aim: To elucidate the molecular nature of sulfhydryl modification by hydrogen peroxide on type 1 ryanodine receptor (RyRl). Methods: Rabbit skeletal muscle sarcoplasmic reticulum was treated with hydrogen peroxide, then RyRl complex was isolated. The proteins in the complex were analysed by electrophoresis, Weste blot and electron microscopy. Results: (1) Hydrogen peroxide induces inter-subunit cross-linking within the tetrameric RyR1 molecule; (2) in parallel to inter-subunit cross-linking, the RyR1 molecule changes morphology; (3) the chemical and morphological changes are reversible: upon reduction by reducing agents, the RyR1 molecule regains its original state. Conclusion: These findings suggest that the molecular mechanism of RyR1 channe1 activity in sarcoplasmic reticulum regulated by hydrogen peroxide is through inter-subunit cross-linking within the tetrameric RyR1 molecule, which in tu induces structural changes of RyR1.