The uptake, transport, storage and excretion of zinc has been studied in Mytilus edulis. Zinc accumulates in the soft tissues in proportion to its concentration in sea water whilst the concentration in the haemolymph is little above that in the environment. Uptake is via the gut, mantle and gills. The zinc is transported from the gills and gut (t½ ≈ 8 days) via the haemolymph, either as a high molecular weight complex or in the granular amoebocytes, to the kidney. Most of the body zinc is present in the granular amoebocytes (which are found in all the body tissues) or in the gut and kidney. The kidney forms the major storage organ for many trace metals, containing 30% of the body zinc and a concentration of about 1000 μg/g. Zinc is localized as insoluble granules in membranelimited vesicles occupying some 20% of the cell volume. Excretion of zinc is by defaecation, exocytosis of the kidney granules into the urine and diapedesis of the amoebocytes. A multicompartmental model for zinc metabolism which correlates the ultrastructural and kinetic data is proposed.