I t is a commonly accepted fact that calcium ions, apart from regulating the permeabil-ity oi cell membranes to various ionic species, determine the level of membrane lec-trical potential at which nerve fibers fire impulses. Since these properties ascribed to Ca ++ seem to be of great importance, a great deal could be gained by having detailed information regarding the permeability of nerve fibers to Ca ++, the physical state of intracellular calcium, or, in general, any chemical reaction in which nerve fiber con-stituents and calcium ions participate. The techniques of microinjection and internal perfusion used in this work have been shown previously to work successfully in the giant axon of Dosidicus (1-3). Only results obtained in nerve fibers whose electrical properties were undamaged through the whole experiment were considered. In the microinjection technique, action poten-tials were recorded only with external electrodes, whereas in the internally perfused fibers the additional control of internal recording was also effected. Stimulating elec-trodes were externally applied in both techniques. The details of the methods have been (4) and will be published elsewhere
