We studied the change of spatial and temporal response properties for cat horizontal (H-) cells during prolonged dark adaptation. H-cell responses were recorded intracellularly in the optically intact, in vivo eye. Spatial and temporal properties were first measured for light-adapted H-cells, followed by a period of dark adaptation, after which the same measurements were repeated. During dark adaptation threshold sensitivity was measured at regular intervals. Stable, long lasting recordings allowed us to measure changes of sensitivity and receptive field characteristics for adaptation periods up to 45 min. Although cat H-cells showed no signs of dark suppression or light sensitization, they remained insensitive in the scotopic range, even after prolonged dark adaptation. Absolute thresholds were in the low mesopic range. The sensitization was brought about by a shift from cone to rod input, and by substantial increases of both spatial and temporal integration upon dark adaptation. The length constant in the light-adapted state was on average about 4 deg. After dark adaptation it was up to a factor of three larger, with a median ratio of 1.85. Response delays, latencies and durations for (equal amplitude) threshold flash responses substantially increased during dark adaptation.