In this study, the distribution of synapses in the ventral centrifugal horizontal (VCH)-a nonspiking, inhibitory, motion-sensitive interneuron in the third visual ganglion (lobula plate) of the blowfly Calliphora erythrocephala-was examined by electron microscopy and electrophysiology. The frequency histograms of excitatory and inhibitory postsynaptic potential amplitudes recorded from the VCH during contralateral stimulus presentation suggest the existence of three neurons, two excitatory and one inhibitory, mediating contralateral input. To localize input and output regions of the VCH, we investigated the ultrastructure of its two arborisation areas after intracellular iontophoretic injection of horseradish peroxidase. The VCH has input synapses in its arborisation in the protocerebrum and in its arborisation in the lobula plate. Output synapses were found exclusively in the lobula plate. Thus, the large dendritic arbor of the VCH in the lobula plate serves simultaneously as an input and an output region. There, we found input and output synapses in close vicinity (0.5-1.5 microm) to each other. Taking into account that the VCH receives retinotopicly arranged input from the ipsilateral eye in the lobula plate, the close location of input and output synapses in the VCH suggests that the spatial organization of its retinotopic synaptic input is more or less conserved in its inhibitory output pattern. The VCH has been proposed to inhibit the figure detection 1 (FD1), another neuron of the lobula plate, that responds preferentially to small moving objects. These results suggest that the FD1 may receive inhibitory inputs from the VCH in the lobula plate, where the dendritic arbors of both neurons overlap.