The structure and synaptic relationships of an intersegmental interneurone in the mesothoracic ganglion of the locust were studied by intracellular labelling for light and electron microscopy. Before labelling, the interneurone was characterized physiologically. It is depolarized and produces spikes when either of the two forewings are elevated, and by sensory inputs from the tarsi of the middle legs. The gross morphology of the interneurone is quite distinctive. Its cell body is on the ventral surface of the ganglion near the midline and its primary neurite lies in the T-tract. Its axon performs a characteristic lateral loop around the lateral dorsal tract before entering an anterior connective and ascending at least as far as the prothoracic ganglion. Within the mesothoracic ganglion, the interneurone gives rise to a profusion of branches. Ultrastructural examination and reconstruction of three regions of the interneurone from serial sections shows that input and output synapses are present in approximately equal numbers, and often are close to each other. There is no evidence for separate input and output regions. Output synapses are characterized by the presence of a presynaptic bar and a population of 500-2,000 round, electron-lucent vesicles. Most of the estimated 10(5)-10(6) synapses occur on the smaller diameter processes or on spines. The spines are of uniform diameter along their lengths, and are pre- or postsynaptic or both, but the postsynaptic spines are of smaller diameter than the others.