1. These experiments examined the effects of a high NaCl diet on (Na+, K+)-ATPase in kidney, heart and cerebral cortex, on the level of circulating inhibitor of (Na+, K+)-ATPase in plasma, and on stimulation of (Na+, K+)-ATPase by treatment with dextro (d)-amphetamine.
2. High salt diet increased indices of (Na+, K+)-ATPase activity (K+-activated p-nitrophenylphosphatase activity and ouabain binding) in kidney medulla, prevented stimulation by amphetamine in cerebral cortex and reduced amphetamine stimulation in heart.
3. High NaCl feeding increased the plasma level of circulating inhibitor of (Na+, K+)-ATPase.
4. Amphetamine alone had no effect on inhibitor level but amphetamine administration reduced the increase in inhibitor with high NaCl feeding.