Whole-cell patch-clamp recordings were obtained from on-bipolar cells in, or isolated from, retinal slices prepared from dogfish retina. The properties of the cGMP-activated conductance of on-bipolar cells were compared with that of rod photoreceptors. The on-bipolar cell cGMP-activated channel was blocked by L-cis-diltiazem, a block which was strongly voltage dependent. However, this channel is not identical with that of photoreceptors. The location of the L-cis-diltiazem blocking site and its accessibility in the channel are not the same as in rods. The voltage dependence of block suggests that the blocking site, although near the intracellular side of the channel, is accessible to the positively charged form of L-cis-diltiazem only from the outward facing side of the channel. Furthermore, in contrast to rod channels, the conductance of the on-bipolar cell channels is unaltered by the removal of external divalent cations.