A GLC method has been developed to measure chlorophenoxyisobutyric acid in plasma after benzol extraction and transformation to its methyl ester, using methyl laurate as the internal standard. Plasma free fatty acids did not interfere with the analysis, but it could not be employed in the presence of salicylates. The method has been employed to study the pharmacokinetics of chlorophenoxyisobutyric acid in man after oral ingestion of a single dose of clofibrate.