To light illumination retinal amacrine cells respond with graded depolarization accompanied by a spike discharge. It has been assumed that the graded depolarization is produced solely by the excitatory synaptic inputs from bipolar cells. Here we demonstrate that a tetrodotoxin (TTX)-sensitive persistent current also contributes to the graded depolarization. This current was isolated in amacrine cells of the goldfish retinal slice preparations by the whole-cell patch clamp technique. The activation threshold of the persistent current was about -50 mV, approximately 10 mV more positive than the mem bra ne potential in the dark. Therefore, it is high ly likely that the TTX-sensitive persistent current is a booster of the excitatory postsynaptic potential in amacrine cells. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.