Isometric tension and aequorin light were recorded from isolated myofibrillar bundles (diameter 0.2 mm) of barnacle muscle fibres to examine Ca release from the sarcoplasmic reticulum (SR). Transfer of a bundle from a pCa 6.7 solution to a pCa 5.8 solution, both buffered with 0.1 mM EGTA, resulted in a phasic increase in myofibrillar free Ca2+ which was superimposed on a slow rise to a steady level and a fast rise in tension. The peak of the free Ca2+ response was higher than the free Ca2+ in the bulk solution. Treatment of the bundle with the detergent Brij to destroy the SR membranes abolished the phasic rise in Ca2+ and considerably reduced the amplitude of contraction. A second challenge of a bundle to the pCa 5.8 solution without prior reloading of the SR Ca store gave a much reduced phasic component. When a pCa 5.8 solution with 1.0 mM EGTA buffering was used, the phasic rise in myofibrillar free Ca2+ could not be detected and the rise in tension was four times slower than with 0.1 mM EGTA. The results are consistent with the operation of a Ca-induced Ca release mechanism in the SR membrane of this crustacean muscle.