Light responses of cone- and rod-driven horizontal cells were recorded intracellularly from opened eyes of intact, immobilized carp and the effects of prolonged darkness on these cells were examined. When the retina was left in the dark, responses of cone horizontal cells to a moderate test flash presented once every 10 min gradually and steadily decreased in size (dark suppression effect). Following a repetitive presentation of the test flash, the light responsiveness of the cells was enhanced. In contrast, the rod horizontal cells did not show the dark suppression effect and the light responsiveness of these cells increased in the dark. Effects of prolonged darkness on the spectral sensitivity of L-type and R/G-type cone horizontal cells were also studied. The spectral sensitivity of L-type cells, determined just after background illumination was extinguished, agreed reasonably well with that of red-sensitive (R-) cones, but it matched better that of green-sensitive (G-) cones when determined in prolonged darkness. We further show that depolarizing responses of R/G-type cone horizontal cells to long-wavelength flashes, driven by R-cones, were abolished after prolonged darkness. Taken together, these results suggest that input from R-cones converging onto the cone horizontal cells is more significantly suppressed after prolonged dark adaptation than input from G-cones. Due to the suppression of the R-cone input, enhancement of responses of the L-type cone horizontal cells to long-wavelength stimulation in the presence of a green background light was no longer seen after prolonged darkness.(ABSTRACT TRUNCATED AT 250 WORDS)