In the isolated amphibian and mammalian cornea, the maintenance of corneal transparency and deturgescence is in part dependent on the rate of net transepithelial Cl transport. Two potent classes of stimulators of this transport process are cyclic AMP mediating agents and the Ca ionophores. These effects coupled with the characterization of high affinity Ca stimulated Mg dependent ATPase/Ca pump activities are part of the basis for suggesting that changes in intracellular cyclic AMP content and Ca activity both serve a second messenger function in eliciting stimulus-secretion coupling. The overall aim is to continue our characterization in the mammalian and amphibian corneal epithelium of the interrelationships and interdependencies among changes in intracellular cyclic AMP content and Ca activities in their possible control of various cell functions. The approaches to this problem are biochemical and electrophysiological in nature. The biochemical approach is at the cellular and subcellular level and its purpose is to obtain insight about: (1) How intracellular Ca activity is maintained at submicromolar levels, which is a requirement for Ca to have a second messenger function; (2) Which metabolic pathways are sensitive to changes in intracellular Ca as well as the mechanism whereby this second messenger exerts its regulatory effects on these pathways; and (3) Any interrelationships and interdependencies among changes in cyclic AMP and Ca levels in mediating their control over these pathways. The electrophysiological approach is concerned with the characterization of the mechanisms of regulation of transepithelial net ion transport. With this approach, it is possible to determine: (1) Which membrane parameters are affected by changes in intracellular cyclic AMP and Ca levels; (2) The stoichiometry of Na:K pumps and symports: (3) Putative cell volume changes in response to alteration of bathing solution tonicity, and (4) If there is a relationship among cell-to-cell coupling and changes in intracellular Ca and pH.