A gas chromatographic determination method of 5-chloro-7-iodo-8-quinolinol (CF), its glucuronide (CF-G) and sulfate (CF-S) in serum, urine and milk was developed. 5, 7-Dichloro-8-quinolinol and/or 5-chloro-7-bromo-8-quinolinol, and their glucuronides and sulfates were added to the sample as the internal standards. Successive extractions with pyridine-benzene (1 : 9) without treatment, after hydrolysis with β-glucuronidase and that with 1 N HCl at 40°, gave the fractions of free quinolinols, derived from the unconjugated, the glucuronides and sulfates, respectively. The free quinolinols were acetylated and determined by gas chromatography. The present method was a demonstration of an effective use of structurally similar internal standards for determination of a glucuronide and sulfate.