1. Removal of both lobes of the pituitary gland, or of the anterior lobe (i.e. pars anterior and pars tuberalis) alone lead to a 22 per cent. fall in the potassium content of the serum.
2. Subsequent regeneration of the pars tuberalis has no effect on the lowered serum potassium level.
3. It is suggested that the pars anterior is the main constituent of the pituitary controlling, directly or indirectly, the potassium content of the serum.