Serum bile acid concentrations were followed in 29 patients with cholestasis of pregnancy treated for pruritus with either phenobarbital or cholestyramine. The response of serum cholic and chenodeoxycholic acid levels to phenobarbital administered to 10 patients in a 100 to 150 mg. daily dose was variable: some decrease was found in four and some increase in three of the patients, but no clear effect on itching could be seen in any of them. In five of the other 10 patients treated with cholestyramine, 4 Gm. two or three times a day, serum cholic and chenodeoxycholic acid levels fell within 1 week of treatment, in one of these near to normal with complete relief from pruritus. It seems that patients with initially high serum bile acid levels do not respond to cholestyramine treatment. In these cases the biliary excretion and enterohepatic circulation of bile acids may be diminished to such an extent that the anion-binding resin is unable to augment fecal elimination of bile salts sufficiently to lower their serum levels.