The time course of recovery from chromatic adaptation in human vision was tracked by determining the wavelength of light that appears uniquely yellow (neither red nor green) both before and after exposure to yellowish green and yellowish red adapting lights. Recovery is complete within 5 min after steady light exposure. After exposure to the alternating repeated sequence 10-sec light/b-sec dark, the initial magnitude of the aftereffect is reduced but recovery is retarded. The results are interpreted in terms of two processes located at different levels in the hierarchical organization of the visual system. One is a change in the balance of cone rece tor sensitivities the second is a shift in the equilibrium base~hne between opposite-signed responses of the red/green channel at the opponent-process neural level. The basemine-shift mechanism is effective in the condition in which repeated input signals originating at the receptors are of sufficient strength to activate the system effectively. Hence, this process is revealed in the alternating adaptation condition when the receptors undergo partial recovery after each light exposure, but receptor adaptation during continued steady light exposure effectively protects the subsequent neural systems from continued strong activation.A salient characteristic of the visual system is its adaptability. It adapts to the level of ambient light, which accounts for the broad (orders of magnitude) range of light levels within which useful vision is possible, and also to the spectral weighting of the ambient light, which accounts for the relative stability of the world of colored objects when seen in various artificial and natural conditions of illumination. The changes that occur during light exposure and the recovery to the preexposure state after light stimulation can be assessed for human observers by standard psychophysical techniques appropriate for threshold estimates of visual sensitivity or by suprathreshold measures based on some constant criterion of visual response. Such measurements have shown, in earlier studies, that visual adaptation can be attributed in part to the retinal receptors and their photosensitive pigments which bleach upon absorption of light quanta and regenerate when the flow of incident quanta is diminished (1, 2). However, retinal receptor adaptation alone cannot account for all of the phenomena attendant upon exposure to different levels and spectral distributions of adapting lights. Also implicated are the neural levels (in the retina and possibly also more centrally in the visual system) where excitatory-inhibitory interactions occur, in neural systems that are laterally interrelated, and in temporal rebound or feedback processes (3-6).Our own earlier studies were concerned with relating different steady-state adaptation conditions, whether to different levels of light exposure or to different spectral distributions of adapting light. The study reported here is concerned with the time course of recovery to a neutral equilibrium conditio...