The process of light adaptation in cat horizontal cells was studied by means of intracellular recordings in the optically intact, in situ eye. Response vs intensity profiles were measured using increment- as well as decrement-flashes of "white light" on steady backgrounds. The effects of background illumination in the mesopic and photopic range on both purely rod-driven horizontal cells and mixed rod and cone input horizontal cells were investigated. Increasing the background illumination for mixed-input horizontal cells strongly reduced the contribution from the rod system. The rod aftereffect in the responses to high intensity flashes is totally suppressed at higher background levels. Light adaptation resulted in a sustained hyperpolarization without a substantial effect on the total response range of the response vs intensity curve. At higher background intensities more of the response range is made up of depolarizing responses to decrements of light. Increasing the background illumination also shifted the operating curve to higher intensities. Increment threshold functions, measured with a 3.9 deg diameter test spot on a large background (8.8 deg diameter) showed a linear relation between log threshold intensity and log background intensity with a slope of, on average, 0.64. The response vs intensity curve for the rod horizontal cell typically spanned a narrower intensity range and was displaced toward lower intensities as compared to that for the mixed input horizontal cells. Background illumination greatly reduced the total response range for rod horizontal cells. Increment threshold curves for rod horizontal cells clearly indicated loss in sensitivity due to response compression.