The interaction of excitatory and inhibitory inputs to the accessory optic system was studied with whole-cell recordings in the turtle basal optic nucleus. Previous studies have shown the visual patterns, drifting in the same preferred direction, evoke excitatory and inhibitory post-synaptic events simultaneously (Ariel and Kogo 2001). Analysis of the reversal potentials for these events and their pharmacological profile suggest that they are mediated by AMPA and GABA_A receptors, respectively (Kogo et al. 2002). Here, neurons were recorded to study non-linear interaction between excitatory and inhibitory responses evoked by electrical microstimulation of the retina and pretectum, respectively. The responses to coincident activation of excitatory and inhibitory inputs exhibited membrane shunting in that the excitatory response amplitude, adjusted for changes in driving force, was attenuated during the onset of the inhibitory response. This non-linear interaction was seen in many but not all stimulus pairings. In some cases, attenuation was followed by an augmentation of the excitatory response. For comparison, the size of the excitatory response was evaluated during a hyperpolarizing current pulse that directly modulated voltage-sensitive channels of a slow rectifying Ih current. Injection of hyperpolarizing current did not cause the attenuation of the excitatory synaptic responses. We conclude that there is a non-linear interaction between these excitatory and inhibitory synaptic currents that is not due to hyperpolarization itself, but probably is a result of their own synaptic conductance changes, i.e. shunting. Since these events are evoked by identical visual stimuli, this interaction may play a role in visual processing.