Recordings from single axons of retinal ganglion cells in the rat's optic tract in response to small flashing test lights were used to follow the course of dark adaptation after exposing half of the receptive field to a bleaching light. 2. The recovery of log sensitivity followed an exponential time course in the exposed and unexposed half-fields. The curves had different time constants, with the exposed side taking longer to recover. 3. The time constants of recovery were increasing functions of exposure, but the rate of increase was different in the exposed and the unexposed half-fields. Direct exposure increased the time constant at a greater rate than did indirect exposure. 4. Comparison of the time constants of recovery in the exposed half-fields with those for pigment regeneration suggests that sensitivity recovers with the time course of rhodopsin regeneration. 5. Increment thresholds were determined using steady backgrounds which illuminated half of the receptive field. A greater threshold elevation was produced in the directly illuminated half-field compared with the half-field illuminated only by scattered light. Comparisons of the threshold-raising capacity of direct and indirect illumination were used to establish an 'upper bound' on the magnitude of light scatter. The time courses of the recovery of sensitivity after two different bleaches were compared. First, thresholds were measured in the unexposed half-field after a half-field bleach. Secondly the recovery of sensitivity after direct bleaching-exposure to the predetermined scatter upper bound' was measured. Recovery was more rapid in the latter case than the former, thus indicating that adaptation spreads laterally via some process other than light scattering.
