The overall goal of this research is to investigate and understand the role of photopic mechanisms in visual adaptation in general, and dark adaptation in particular. Evidence that spatial integration of light is controlled by a single variable has been demonstrated in the rod or scotopic system. Until recently, however, the conclusions drawm from experiments of scotopic adaptation have been extrapolated to explain adaptation in the photopic system. Experiments involving adaptation in the photopic system have produced complex and sometimes conflicting results, therefore, research on the photopic mechanisms of dark adaptation is indicated. The proposed research is concerned with spatio-temporal integration of light in long-term dark adaptation and in early light and dark adaptation. The processes of adaptation will be investigated mainly in terms of the underlying wavelength-dependent excitatory and inhibitory mechanisms of the receptive field using human psychophysical methods. Appropriate and meaningful values of test-stimulus characteristics will be chosen empirically using the sensitization effect. The Crawford equivalent-background transformation will be used to investigate the process of adaptation using temporally, spatially, and chromatically varying stimuli. Color-defective as well as color-normal observers will be examined in order to more completely isolate and study the different cone mechanisms. An extensive literature and a number of hypotheses have been developed to deal wth the nature of the dark-adaptation process. The information to be obtained from the proposed research should have import with regard to single-variable, bleaching signal hypotheses and receptive-field hypotheses of adaptation, and should allow a more adequate model of visual adaptaion to be developed.