The functional relationship between eccentric and retinular cells within ommatidia of the Limulus lateral eye will be studied by determining (a) the role of retinular cell axons in the excitation of cells in the optic ganglia and (b) the ultrastructure and functional correlates of changes in retinular and eccentric cell morphology during light-dark adaptation. The currents responsible for the production of the receptor potential of retinular cells and in the Limulus lateral eye will be studied in normal ionic environments and in environments deficient in Na ion, Ca ions or K ion and environments rich in Ca ion; or K ion using the voltage clamp technique. The effects of Mn C12 and aminophylline at low concentration (2 micron M or less) on photoreceptor currents will be investigated. Particular attention will be directed at the current responsible for the production of the first component of the receptor potential.