1. S-potentials were recorded in the dark-adapted carp retina. After recording, Procion Yellow was injected from the recording electrode to identify histologically the cell recorded.2. Both L- and C-type S-potentials were found. L-type S-potentials were further classified into two groups. The first group was characterized by a high threshold, fast rise and fall times and a spectral sensitivity peak of about 620 nm. The second group showed a low threshold, slow response time course and a spectral sensitivity peak of about 520 nm.3. Cells of the first group were identified as external and internal horizontal cells. Cells of the second group were identified as intermediate horizontal cells.4. In the dark-adapted retina, the spectral sensitivity of the intermediate horizontal cell showed a good agreement with the absorption spectrum of the rod pigment, porphyropsin.5. Neither a chromatic background nor white background illumination of moderate intensity (1 cd.m(-2)) shifted the spectral sensitivity of the intermediate horizontal cell.6. Under a strong background (10 cd.m(-2)), the intermediate horizontal cell was entirely suppressed. Under the same conditions the external horizontal cell was still capable of responding.7. From these observations it is concluded that the intermediate horizontal cell receives input from rods only.