Selective suppression of rod signal transmission by cobalt ions was reported in carp retina. Using 10 μmol/L Co2+ , rod-driven horizontal cells were hyperpolarized and light responses were completely suppressed in super-fused, isolated retina, while cone-driven horizontal cells were almost unaffected. Similarly, scotopic electroretino-graphic b-wave was suppressed by 10 μmol/L Co2+ , while the photopic b-wave remained unaffected. Furthermore, the glutamate-isolated receptor potential (PIII) was not altered by low Co2+ under dark-adapted conditions. Other di-valent ions with high affinity to calcium channels, such as cadmium and manganese ions, did not show similar suppres-sive effect on the rod horizontal cells. When rod horizontal cells were hyperpolarized by 10 μmol/L Co2+ , the use of 3 mmol/L glutamate caused a significant depolarization of the cells, indicating that Co2+ application did not impair the ability of these cells to respond to glutamate. On the other hand, application of 200 μmol/L ? Selective suppression of rod signal transmission by cobalt ions was reported in carp retina. Using 10 μmol / L Co2 +, rod-driven horizontal cells were hyperpolarized and light responses were completely suppressed in super-fused, isolated retina, while cone-driven horizontal cells were almost unaffected. Similarly, scotopic electroretino-graphic b-wave was suppressed by 10 μmol / L Co2 +, while the photopic b-wave was unUffected. Furthermore, the glutamate-isolated receptor potential (PIII) was not altered by low Co2 + under dark- adapted conditions. Other di-valent ions with high affinity to calcium channels, such as cadmium and manganese ions, did not show similar suppres- sive effect on the rod horizontal cells. When rod horizontal cells were hyperpolarized by 10 μmol / L Co2 +, the use of 3 mmol / L glutamate caused a significant depolarization of the cells, indicating that Co2 + application did not impair the ability of these cells to respond to glutamate. On the other hand, applicatio n of 200 μmol / L?