The long range goal of this research is to describe the factors which control ion transport across the surface membrane of nerve and muscle. Projects proposed for the requested period of support include: 1. Measurement of the concentration of free calcium and magnesium in the cytoplasm of squid axons, barnacle muscle fibers and Myxicola giant axons during resting conditions, electrical or mechanical activity, or during periods of altered metabolism. 2. Study of the divalent ion fluxes (calcium and magnesium) in barnacle muscle fibers, and giant nerve fibers of Myxicola and squid. Special emphasis will be placed upon characterizing the unidirectional fluxes as a function of internal concentration of divalent and other ions, and of transverse membrane potential. 3. Initiate development of apparatus to resolve flux transients across the surface of frog muscle fibers during contraction. Methods include: 1. Control of internal environment by internal dialysis of fibers by a porous plastic capillary. 2. Control of membrane potential by radial current passage. 3. Use of radioactive isotopes. 4. Sensitive differential absorption spectrometry.