Responses of centre-hyperpolarizing, red-sensitive bipolar cells were studied by intracellular recording in the retina of the turtle, Pseudemys scripta elegans. The identity of these cells was confirmed by Procion Yellow marking. 2. Circles of light produced hyperpolarizing waves that were graded with intensity and could exceed -30mV in amplitude. The operating intensity range was similar to that of turtle cones. 3. Flashes in the form of an annulus evoked graded depolarizations which could be greater than 10 mV in the dark-adapted state or about 30mV when applied over central backgrounds. 4. Responses proportional to intensity were produced by dim circular stimuli. For radii less than about 200 mum these responses reached peak in approximately 120 msec and were invariant with respect to wave-length or area of illumination. Absolute flash sensitivity varied greatly from cell to cell but in the most sensitive cell encountered was about 460 muV photon(-1) um2. 5. Sensitivity of both bipolar cells and red-sensitive cones was enhanced progressively for enlargements of a circular flash up to 150-200 mum in radius. 6. Increasing the radius of a circle from 200 to 1250 mum caused a decrease of about 75% in bipolar cell sensitivity. This decrease was associated with a marked shortening of the response for all colours. The same enlargement decreased sensitivity of red-sensitive cones by approximately 20% and did not appreciably alter the time course of their response. These effects are attributed to impingement from type I red-sensitive horizontal cells because they have the requisite spatial and spectral properties. 7. Responses of a few bipolar cells were already shortened for 200 mum flashes; this property suggests impingement from type II horizontal cells. 8. For small circles the spectral sensitivity of the bipolar cells considered resembled closely that of red-sensitive cones or horizontal cells. Red backgrounds enhanced the relative sensitivity to green flashes suggesting that these bipolar cells receive input from red-sensitive members of double cones as well as single red-sensitive cones. 9. Steady depolarizing currents injected into bipolar cells decreased the response to either central or annular illumination; hyperpolarizing currents decreased the response to a central flash and increased the response to an annulus.
