1. The influence of horizontal cells on ganglion cells, the output neuron of the retina, was examined in an in vitro rabbit eyecup preparation. The extracellular spike activity of ganglion cells was monitored while pulsatile DC or sinusoidally modulated current was injected intracellularly into nearby horizontal cells. Interactions between the effects of light stimulation and horizontal cell current injections on ganglion cell responses were also examined. 2. Horizontal cells were found to contribute to the receptive field surround of ganglion cells. In particular, horizontal cells contributed to surround excitability and to surround antagonism of the centre light response. 3. Brisk, sluggish and direction-selective ganglion cells were all affected by current injections into horizontal cells. However, brisk ganglion cells responded to lower amplitude currents than did sluggish or direction-selective cells. 4. Horizontal cells with receptive fields that overlap those of ganglion cells were able to affect ganglion cell discharge. Moreover, the closer a horizontal cell was to the receptive field centre of a ganglion cell, the more effective were current injections in modulating ganglion cell discharge rate. The length constant of the horizontal cell contribution to the ganglion cell receptive field was approximately 200 microns. These results indicate that horizontal cells which are located within or outside of a ganglion cell's receptive field centre can influence that ganglion cell's activity. 5. The influence of horizontal cells on ganglion cell discharges was relatively weak at low temporal frequencies of sinusoidally modulated current. 6. Application of 2-amino-4-phosphonobutyrate (APB), a glutamate analogue, blocked the modulation of spike activity of on-centre ganglion cells that was induced by sinusoidally modulated current injected into nearby horizontal cells. The spike activity of off-centre ganglion cells was not blocked. 7. These findings suggest that horizontal cells contribute to the surround of ganglion cells and bipolar cells primarily through a feedback pathway onto cone photoreceptor cells.