Calcium absorption was investigated in young rats maintained on either high or low levels of protein in the diet and in adult rats, maintained on adequate diet to start with and then switching some on to a low-protein diet, employing in vivo and in vitro techniques. Results of the three techniques employed-(1) serum radioactivity after 1 h of gavage feeding 45CaCl2 solution, (2) 45Ca uptake by duodenal slices after 1 h incubation in the medium containing 45Ca, and (3) everted gut sac technique – indicated that the capacity of the intestine for calcium absorption in rats was not influenced by the level of protein in the die