Rhodopsin, the visual pigment present in the rod photoreceptors of many vertebrates, is centrally involved in the process of visual adaption within the retina. As yet, the mechanisms by which visual pigment influences receptor sensitivity are not well understood. The broad objectives of the proposed research are to elucidate (1) the dependence of rod photoresponses on the content of rhodopsin; (2) the visual activity of artificial rod pigments; and (3) the receptoral reactions of vitamin A during dark adaptation. The principal type of experiment utilizes the isolated, all-rod retina of the skate, and involves the extracellular measurement of the aspartate-isolated, massed photoreceptor potential. Receptor sensitivity and response amplitude will be measured under different conditions of light adaptation, and upon the external application of 11-cis retinal (the chromophore of rhodopsin) and related compounds to the photoreceptors. Analogs of 11-cis retinal will be applied to strongly light-adapted receptors, and the resulting artificial pigments will be examined with regard to their activity in dark adaptation and in phototransduction. For the investigation of receptoral vitamin A reactions during dark adaptation, putative precursors of 11-cis retinal will be applied to strongly light-adapted skate rods, and the ability of the rods to transform these compounds into 11-cis retinal will be monitored electrophysiologically and spectrophotometrically. Photoreceptors in the isolated retinas of other vertebrate species will be similarly examined for their utilization of vitamin A compounds.