The leading edge of the a-wave of the ERG is generally believed to accurately reflect the changes in the circulating current through the cGMP-gated channels in the outer segment plasma membrane of rods and cones. The aspartate-isolated mammalian electroretinogram (ERG) to a rod-saturating flash contains a fast "nose"-like wave temporally overlapping with the a-wave. We characterize the nature of this nose, investigate the membrane current mechanisms involved in the nose mechanism, and propose a model that can explain the generation of the nose component in the rod inner segment. On the basis of pharmacological treatments and perfusate ion composition alterations we rule out the possible role of most of the known rod membrane current mechanisms that might participate in the generation of the ERG nose component and we propose that the nose is generated by the interplay of voltage-dependent K(x) and h channels together with the Na(+)/K(+) ATPase. Our results strengthen the view that the kinetics of the leading edge of the ERG photoresponses should correspond to that of the outer segment light-sensitive current.