We show that rabbit skeletal RyR channels in lipid bilayers can be activated or inhibited by NO, in a manner that depends on donor concentration, membrane potential and the presence of channel agonists. 10 microm S-nitroso-N-acetyl-penicillamine (SNAP) increased RyR activity at -40 mV within 15 sec of addition to the cis chamber, with a 2-fold increase in frequency of channel opening (F(o)). 10 microm SNAP did not alter activity at +40 mV and did not further activate RyRs previously activated by 2 mm cis ATP at +40 or -40 mV. In contrast to the increase in F(o) with 10 microm SNAP, 1 mm SNAP caused a 2-fold reduction in F(o) but a 1.5-fold increase in mean open time (T(o)) at -40 mV in the absence of ATP. 1 mm SNAP or 0.5 mm sodium nitroprusside (SNP) induced approximately 3-fold reductions in F(o) and T(o) at +40 or -40 mV when channels were activated by 2 mm cis ATP or in channels activated by 6.5 microm peptide A at -40 mV (peptide A corresponds to part of the II-III loop of the skeletal dihydropyridine receptor). Both SNAP-induced activation and SNAP/SNP-induced inhibition were reversed by 2 mm dithiothreitol. The results suggest that S-Nitrosylation or oxidation of at least three classes of protein thiols by NO each produced characteristic changes in RyR activity. We propose that, in vivo, initial release of NO activates RyRs, but stronger release increases [NO] and inhibits RyR activity and contraction.