The distribution of ryanodine receptor calcium-release channels over the terminal cisternae (TC) membrane of skeletal muscle fibres was examined by using immunogold electron microscopy. Two monoclonal antibodies (5C3 and 8E2) that bound to monomers of the ryanodine receptor protein on Western blots of SDS-polyacrylamide gels were used to locate calcium-release channels in longitudinal sections of rat sternomastoid and diaphragm fibres. Up to 21% of 5C3 binding on TC membranes was extra-junctional, compared with 46% for 8E2. Binding of 8E2 to the fibres was less than half that of 5C3, possibly because of steric shielding of the 8E2 antigenic site at the junction. The distances between neighbouring particles in clusters was 20-40 nm, i.e. the distance between subunits of the ryanodine receptor or between neighbouring foot structures. We suggest that, during activation, extra-junctional ryanodine receptors may release Ca2+ directly into the myoplasm, rather than into the restricted space of the triad junction.