The responses of sustaining and dimming fibers were characterized by the time varying firing rates elicited by extrinsic current and flashes of light. These data were simulated by an adaptive integrate-and-fire model. A post-impulse shunt conductance simulated spike-frequency adaptation. The correlation between observed and model current-elicited impulse rates was 0.94 to 0.98. But for a difference in input resistance,(both measured and simulated)the voltage to impulse encoders of the two cell groups are similar and exhibit comparable degrees of spike frequency adaptation (40 to 45%). The encoder model derived from current-elicited responses (with fixed parameters) was used to simulate visual responses elicited by light flashes. These simulations included a synaptic current derived from the time course of the postsynaptic potential (PSP). The sustaining fiber visual response consists of a large excitatory PSP and high frequency transient burst which adapts (by about 80%) to a low frequency plateau discharge. The simulations indicate that spike-frequency adaptation has no effect on the transient discharge but reduces the plateau firing rates by about 60%. Encoder adaptation enhances the sustaining fiber response to the time derivative of the stimulus. In dimming fibers, the light flash elicits an inhibitory PSP which interrupts the 'dark discharge' and an OFF response following the end of the flash. The simulations indicate that spike-frequency adaptation reduces the firing rate of both the dark discharge and the OFF response. Thus the model suggests that different effects of encoder adaptation on the two cell types arise from the same encoder mechanisms but with different actions determined by differences in impulse rate and the time course of the discharge.