There are five morphological types of photoreceptors in the retina of the toad, Bufo marinus: red and green rods, single cones, and the principal and accessory members of double cones. The largest and most abundant of these is the red rod. 2. Intracellular recordings were used to investigate the dependence of the sensitivity of red rod responses on wave-length and background light. 3. The spectral sensitivity of dark-adapted and moderately light-adapted red rods can be satisfactorily fitted with the absorbance spectrum of the red rod photopigment. There are no significant contributions to red rod responses from cones or green rods. 4. In contrast, L-type horizontal cells, whose responses are dominated by input from the red rods near threshold, can be shown also to receive input from cones. 5. Steady background light produces a response in the red rods consisting of an initial hyperpolarization, followed by a decay of potential to a steady-state plateau level. The slow decay of response amplitude is accompanied by an increase in sensitivity to increment test flashes. 6. The increment sensitivity at steady-state decreases with increasing background intensity according to a modified Weber-Fechner relation. The dependence of increment sensitivity on the wave-length of the background light can be predicted by the red rod spectral sensitivity, showing that cones do not influence the light adaptation of rods. 7. At a background [corrected] intensity of 11-5 log equivalent quanta cm-2sec-1, sensitivity begins to deviate from the Weber-Fechner relation. In background light one log unit brighter, the rods are completely saturated. 8. Small responses having the spectral sensitivity of cones can be recorded from saturated rods. These potentials have a prominent off response whose wave form resembles the d-wave of the e.r.g. 9. A comparison of the increment--sensitivity curves of single receptors shows that rods are light-adapted by backgrounds one thousand times dimmer than those which affect cones. The increment--sensitivity curves of rods and cones cross, so that single cones become more sensitive than single rods even before the rods begin to saturate.
