Quantitative light microscope autoradiography of 3H-ouabain, a specific inhibitor of Na ion - K ion ATPase, will be used to measure the distribution of Na ion - K ion pump sites in cone photoreceptors of turtle and lizard. The objective is to determine the number of sites/receptor and the relative distribution between outer segment and proximal receptor. Development of EM autoradiography of ouabain binding in tissue processed by freeze-drying and vacuum embedding will be attempted to answer the latter question. Work will also be continued on synthesis of a covalently reactive cardioglycoside with the goal of developing a pump site marker which permits autoradiography of conventionally fixed tissue.