1. Zinc concentrations in the freshwater crayfish Austropotamobius pallipes pallipes normally range from about I µg./g. in the blood to 100 µg./g. in the hepatopancreas.
2. The permeability of the body surface to zinc is very low. Long exposure to concentrations exceeding that of the blood is required to increase the internal tissue zinc concentrations appreciably.
3. Much of the zinc which is absorbed from solution appears to be adsorbed on to the gill and shell surfaces.
4. Most of the body zinc is obtained from food.
5. The hepatopancreas is the principal organ of zinc regulation. It can absorb excess zinc from the stomach fluid and can remove excess zinc if this is injected into the blood.
6. Very little of the excess zinc in the hepatopancreas can be lost in the urine or across the body surface. Zinc is lost only when the animal feeds and faeces are produced to which it can bind.
7. As the amount of zinc in the food increases, a smaller percentage of it is absorbed by the hepatopancreas and more is lost in the faeces.
8. Regulation of zinc seems to depend on changes in the hepatopancreas/stomach-fluid ratio. These alter the availability of zinc for removal in the faeces according to the concentration in the hepatopancreas.
9. There is no close relationship between the behaviour of zinc and copper although zinc is bound to blood proteins some of which are haemocyanins.
10. Differences in the methods of regulation between the freshwater crayfish and the marine lobster may represent changes which have occurred during the penetration of the crayfish into fresh water.