1. Rat liver nuclei were isolated from normal rats and rats fasted for 36 hours by a slight modification of the Behrens technique.
2. The nucleus of the rat liver cell contains two types of alkaline phosphatase. This confirms the previous findings on rat liver nuclei isolated in aqueous media.
3. The one type of alkaline phosphatase is not activated by magnesium ions, and this enzyme is very strongly bound to structural material of the nucleus. The other type of alkaline phosphatase is activated by magnesium ions, and this enzyme is probably free to diffuse from cytoplasm to nucleus and vice versa through the nuclear membrane.
4. Fasting caused a pronounced decrease of protein in general and of the alkaline phosphatase which is activated by magnesium ions from the nucleus of the rat liver cell, while the alkaline phosphatase that is not activated by magnesium was less affected.