We examined the effects of hydroxyl radical on the ion permeability of the ryanodine receptor, a calcium releasing channel of sarcoplasmic reticulum. The cardiac ryanodine receptor, purified from pig heart, was reconstituted to proteoliposomes and then incorporated into a planar bilayer membrane. A single channel activity with a conductance of 724 pS in 900/200 mM (cis/trans) KCl and an ion selectivity of P_K:P_Cl= 1:0.08 was observed. These characteristics are similar to those observed by the incorporation of the channel through sarcoplasmic reticulum vesicles. Hydroxyl radicals chemically generated by the reaction of H₂O₂and Cu(ethylenediamine)₂at theciscompartment increased the open probability of the channel. Treatment with SH oxidizing reagents from theciscompartment also increased the open probability, and dithiothreitol, a SH reducing agent, reversed the effect. These findings suggest that hydroxyl radicals react with some SH groups of the ryanodine receptor and increase the Ca²⁺release from sarcoplasmic reticulum through the ryanodine receptor.