Rapid calcium efflux from the sarcoplasmic reticulum (SR) is a necessary step in excitation-contraction coupling in skeletal muscle and is thought to be mediated by a calcium channel. Calcium efflux has been studied in fragmented SR vesicles by radioisotope efflux and fluorescence measurements. Several laboratories have reported that adenine nucleotides can stimulate calcium efflux from SR. In recent reports, Ca2+ release with a first-order rate constant as high as 100 s-1 has been observed for nucleotide-stimulated Ca2+ release from SR vesicles. Also, radioisotope efflux was blocked by Mg2+ and micromolar concentrations of the polycationic dye, ruthenium red. These high rates of transport are difficult to reconcile with a mechanism other than passive diffusion through a nucleotide-activated 'calcium release channel'. Using the fusion technique for inserting SR proteins into planar lipid bilayers, we report here single-channel recordings of calcium release channels from purified 'heavy' SR membranes. Channels have been identified on the basis of their activation by adenine nucleotides, blockade by ruthenium red, and selectivity for divalent cations. Surprisingly, the channel studied here exhibits an unusually large conductance of 170 pS in 50 mM Ba2+ while still being capable of discriminating against monovalent cations by a permeability ratio, P(Ba)/P(Cs) = 11.4.