Duodenums from freshly killed stock colony rats were incubated in Krebs buffer containing carrier-free 65ZnCl2 for periods of time between 5 and 60 s. Mucosa cytosol was separated and fractionated by gel filtration on Sephadex G-75.65Zn was present in two main bands of protein with molecular weights of about 45,000 and 6,500, but the ratio between them rose progressively with time of exposure to the isotope. Addition of metabolic inhibitors greatly lowered the 45,000/6,500 ratio for 65Zn. It is concluded that when zinc enters the mucosal cytoplasm during absorption, it rapidly binds to the 6,500 protein in a passive way and is then transferred to the protein of molecular weight 45,000 by an active process.