The present report demonstrates the effect of Cd on intestinal Ca transport and Ca-binding protein in rats adapted to vitamin D repletion and high Ca. There were significant decreases in in vitro intestinal Ca transport in animals exposed to Cd for 2 or 3 months as compared with controls. Ca transport from everted gut sacs of normal rats showed a dose-dependent decrease. 45Ca-binding activity to intestinal mucosa from Cd-exposed rats showed a lower level of decrease than that on control rat mucosa. The typical peak of Cd-binding protein obtained from Sephadex G-100 gel filtration of 109Cd added to mucosal supernatant agreed with results of gel filtration obtained after 45Ca had been added to the same supernatant. Although there were 2 proteins binding Cd or Ca in the intestinal mucosa, with the same molecular weight of 10,000, these proteins were shown to be different, since Cd did not bind with the Ca protein. It was concluded that inhibition of vitamin-D-non-stimulated Ca transport was not caused by Cd bound to Ca-binding protein, but by some other factor.