This study investigated the influence of hyperthyroidism on intestinal Ca absorption with an in vivo perfusion technique in the rat duodenum. Net movement, plasma to lumen and lumen to plasma fluxes, although slightly decreased, were not statistically significantly different in hyperthyroid animals compared to normal animals. These results do not support the theory of decreased Ca absorption in the duodenum as a major factor contributory to the increased fecal excretion of Ca seen in hyperthyroid rats.