1. In sea-water, adult salmon (S. salar) exchange an average of 12.6% of total body sodium/hr. 2. Following transfer to fresh water sodium uptake follows Michaelis-Menton kinetics. Fmax = 2.40 mmol Na/1 ECF/hr, Km = 0.26 mmol Na/1. The uptake system is fully activated immediately following transfer to fresh water. 3. Post smolts adapted to sea-water for 3 months take up sodium at only one third of the rate of adult fish following return to fresh water. 4. The concentration of prolactin in the plasma is low in sea-water adapted fish and does not rise during the first 8 hr in fresh water. 5. At pH 5 sodium uptake is reduced by almost 90%, even in the absence of aluminium, but recovers immediately on return to neutral water. 6. At pH 5 and 20 μmol Al/1 there is little further effect on sodium uptake but after 6 hr in aluminium the inhibition of sodium uptake continues after return to neutral aluminium fresh water and uptake is only 50% of normal 24 hr later.