Fasting and postprandial serum bile acid concentrations were determined by gas-liquid chromatography in 20 consecutive individuals (14 normal subjects, 6 cirrhotics) before and after administration of rifampicin in a single dose of 900 mg, using each individual as his own control. In the normal subjects the 2-hr postprandial level was 2.9 +/- 0.2 microM (mean +/- 1 SEM) prior to drug administration. Following rifampicin, it was 7.7 +/- 0.5 microM (P less than 0.0005). In the patients with liver cirrhosis the 2-hr postprandial level increased from 30.2 +/- 3.5 before, to 105.0 +/- 27.7 microM after rifampicin (P less than 0.025). Twelve hours after drug administration the levels of serum bile acids were not significantly different with respect to the control values. Total serum bilirubin also showed a transient and parallel increase. These findings are compatible with the hypothesis that rifampicin inhibits the hepatic transport of bile acids.