The cardiac and skeletal muscle sarcoplasmic reticulum ryanodine receptor Ca²⁺ release channels contain thiols that are potential targets of endogenously produced reactive oxygen and nitrogen intermediates. Previously, we showed that the skeletal muscle ryanodine receptor (RyR1) has O₂-sensitive thiols; only when these thiols are in the reduced state (pO₂ ∼ 10 mmHg) can physiological concentrations of NO (nanomolar) activate RyR1. Here, we report that cardiac muscle ryanodine receptor (RyR2) activity also depends on pO₂, but unlike RyR1, RyR2 was not activated or S-nitrosylated directly by NO. Rather, activation and S-nitrosylation of RyR2 required S-nitrosoglutathione. The effects of peroxynitrite were indiscriminate on RyR1 and RyR2. Our results indicate that both RyR1 and RyR2 are pO₂-responsive yet point to different mechanisms by which NO and S-nitrosoglutathione influence cardiac and skeletal muscle sarcoplasmic reticulum Ca²⁺ release.