We have studied the regulation or homeostasis of cholic acid output in biliary-duodenal-suction-fistula dogs under the following 3 conditions: a) When the initial dose was only 0.46 g. (the basal output per 8-hr. period, when no bile is returned); b) when the initial dose was 2.7 g.; and c) when the initial dose was 5.7 gm. The dogs were fed J of the standard every 8 hrs., and the bile circulated through 3 entero-hepatic circulations each day. Regardless of the initial dose, the cholic acid output tended to reach a homeostatic level ranging between 3.7 gs. and 4.1 gs. per 8-hr. period. When large doses of cholates were administered, the curve of cholic acid fell to the homeostatic level, whereas, with small doses, such as 0.46 gs. and 2.7 gs., the curve rose to the homeostatic level. When fed our standard diet, the animals synthesized from 0.46 to 0.48 gs. per 8-hr. period, and when a certain amt. of cholates were administered, on the avg., 88% was recovered in the bile. Thus at the homeostatic level, synthesis apparently balances "loss" of cholic acid. An equation is presented which permits the prediction of the cholic acid output for any subsequent period when the basal output per unit of time on the diet is known, and the avg. loss of cholic acid during one entero-hepatic circuit is known and the diet and frequency of feeding are kept constant.