The giant-fibre responses of Harmothoe and Nereis have been studied with emphasis on the afferent and efferent pathways and the sites of the rapid accommodation of the fast response. When the bundle of sensory neurones of the anal cirrus is stimulated the giant fibres respond at the first shock. This terminal junction then rapidly accommodates to further afferent excitation. In Harmothoe the muscles which effect the rapid movements of the giant-fibre response are directly innervated by large axons of unipolar cell bodies in the central nervous system. In each segment one neurone of this type supplies both dorsal and ventral longitudinal muscle fibres. By use of a bridge technique this neurone has been isolated. At the first stimulus above a single sharp threshold the resulting nerve muscle preparation gives a maximum electrical response, which is independent of the stimulus strength. A reduced response persists for many repetitions at low frequency. In addition to this fast motor neurone a slower system in the same muscles is indicated by the muscle-action potentials and by observation of the movements. An axon-axon synapse seen histologically between the lateral giant fibres and the large motor neurone to the longitudinal muscle has been identified with the rapidly accommodating physiological junction between these elements. At this level of analysis the afferent and efferent relations of the giant fibres, and in particular the fast motor innervation, are broadly comparable with those of some arthropods.