This paper is concerned with a re-examination of the pattern of nerve activity from receptors in ray fin muscles in response to stretch applied to the muscles. In earlier work receptors have been found in the fin muscles of elasmobranchs, those in rays being described in a recent paper by Bone & Chubb (1975 - which see for references to earlier anatomical work) and physiologically by Fessard & Sand (1937). The receptors are supplied by myelinated axons, and the endings take the form of beaded nerve strands associated with congregations of connective tissue lying between muscle fibres. In the pectoral fin muscles these structures lie between small-diameter muscle fibres, with multiple motor nerve endings, which occur superficially in the muscle. Most of these muscle fibres show a positive staining reaction for succinic dehydrogenase (SDH) and lipid, though some are negative for SDH. They contrast with the larger fibres, which show negative staining reactions for SDH and lipid, andhave focal motor innervation. The latter make up the bulk of the muscle (Bone & Chubb, 1975). It seems very likely that most of the small fibres are slow contracting (possibly with no propagated action potential in the muscle fibre), and the large fibres are of a twitch type. This would parallel the arrangement in dogfish myotomes (Bone, 1966). Bone & Chubb consider the coupling between the endings and the muscle fibres to be generally loose, although they also describe occasional sites of much tighter coupling. Such small muscle fibres are particularly concentrated in a superficial and rather discrete part of the muscle, here called the tonic bundle and used in the present experiments as an isolated preparation.