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1. Dialysed giant axons from the squid have been used to study some of the properties of the Na+ fluxes when the Na+ pump is fully inhibited by strophanthidin. 2. In axons which had been depleted of ATP, strophanthidin had no effect on Na+ efflux. Similar negative results were obtained in axons dialysed with and without internal or external K+, and with or without 100 microM-internal Ca2+. 3. In the presence of 60 mM-internal Na+, 440 mM-external Na+ and strophanthidin, the fluxes of Na+ had the following characteristics. (i) ATP stimulated an efflux and an influx of Na+ of similar magnitude. The K1/2 for ATP, measured from its effect on Na+ efflux, was about 200 microM. (ii) The non-hydrolysable ATP analogue adenylyl(beta, gamma-methylene)-diphosphonate (AMP-PCP), at 2 mM concentration, either alone or in combination with 2 mM-internal phosphate, failed to stimulate any efflux of Na+. (iii) The ATP-dependent Na+ efflux was not affected by removal of internal or external K+, or external Mg2+ or Ca2+, and was not dependent on internal Ca2+. (iv) within the resolution of the method, all the ATP-dependent Na+ influx required internal Na+, and all the ATP-dependent Na+ efflux required external Na+. From the magnitude of the unidirectional Na+ fluxes the stoichiometry seemed to be a 1 to 1 Na+--Na+ exchange. 4. The ATP-internal Na+-dependent influx of Na+ in the presence of strophanthidin was not affected by 1 mM-vandate in the dialysis solution, a concentration which fully inhibits the Na+ efflux through the Na+ pump that is activated by external K+. 5. In the presence of external Na+, the external K+ sites of the Na+ pump are completely saturated with 100 mM-external K+. In unpoisoned axons incubated with 100 mM-external K+, replacement of external Na+ with Tris+ produced no change in the efflux of Na+. However, in axons poisoned with 50 microM-strophanthidin, replacement of external Na+ with Tris+ resulted in a reversible inhibition of Na+ efflux. This could suggest that strophanthidin poisoning might induce Na+ (cations?) fluxes which are not present in normal conditions.