Continuous nondestructive optical techniques, using specific metallochromic dyes; permit measurement of ionized calcium and magnesium concentrations in isolated squid axons and barnacle muscle fibers. With respect to Calcium: It is proposed to relate load in these preparations to ionized calcium with emphasis upon metabolic requirements for calcium buffering. With respect to Magnesium: It is proposed to relate changes in ionized magnesium to metabolic activity in the cell - especially the ATP content.