In the isolated and perfused retina of the toad, intracellular work will be conducted in the outer segments of red rods. The effects of changing ionic composition of the perfusate will be studied, to elucidate the ionic mechanisms that control membrane potential, the light-evoked receptor potental, and receptor sensitivity. Particular attention will be given to Ca2 ion and Ba2 ion, which have been shown to exert strong effects, and which are both indicated by prior evidence to be concentrated in the outer segments of vertebrate photoreceptors. Other ions and substances will also be studied that are normally present, and that have putative effects in generating or modifying the response of photoreceptors. Stringent attempts will be made to apply our newly developed microelectrode techniques to intracellular recording from rods and cones of mammalian retinas. This will be done in arterially perfused eyecup preparations of tree squirrels, which offer good possibilities for both rod and cone recordings. Initial goals will be to study the form and amplitude of receptor potentials as a function of stimulus intensity, and to determine the types of receptor interactions that can be demonstrated. In snapping turtle, simultaneous recordings will be attempted with two miroelectrodes, one in a photoreceptor (either rod or cone) and the other in a horizontal cell. The goal of this work will be to study directly the properties of synaptic transfer between photoreceptors and second order cells of the retina.