The single-channel activity of rabbit skeletal muscle ryanodine receptor (skeletal RyR) and dog cardiac RyR was studied as a function of cytosolic [Ca2+]. The studies reveal that for both skeletal and cardiac RyRs, heterogeneous populations of channels exist, rather than a uniform behavior. Skeletal muscle RyRs displayed two extremes of behavior: 1) low-activity RyRs (LA skeletal RyRs, approximately 35% of the channels) had very low open probability (Po < 0.1) at all [Ca2+] and remained closed in the presence of Mg2+ (2 mM) and ATP (1 mM); 2) high-activity RyRs (HA skeletal RyRs) had much higher activity and displayed further heterogeneity in their Po values at low [Ca2+] (< 50 nM), and in their patterns of activation by [Ca2+]. Hill coefficients for activation (nHa) varied from 0.8 to 5.2. Cardiac RyRs, in comparison, behaved more homogeneously. Most cardiac RyRs were closed at 100 nM [Ca2+] and activated in a cooperative manner (nHa ranged from 1.6 to 5.0), reaching a high Po (> 0.6) in the presence and absence of Mg2+ and ATP. Heart RyRs were much less sensitive (10x) to inhibition by [Ca2+] than skeletal RyRs. The differential heterogeneity of heart versus skeletal muscle RyRs may reflect the modulation required for calcium-induced calcium release versus depolarization-induced Ca2+ release.