A specific and quantitative GLC method was developed for the determination of (2-cyclopentyl-6,7-dichloro-2-methyl-1-oxo-5-indanyloxy) acetic acid, a novel saluretic uricosuric agent, in biological fluids. The procedure involves the addition of an internal standard to the biological specimens followed by extraction of the acids into benzene at pH 1. The extracted acids, following back-extraction into base and reextraction into methylene chloride at an acidic pH, are converted to the respective methyl esters by reaction with ethereal diazomethane. The sensitivity of the method is such that 2 microgram of material can be detected per aliquot of plasma or urine. In the 2.5-50-microgram/ml range, recoveries were 98.1 +/- 9.6% (plasma, n = 157) and 99.3 +/- 6.4% (urine, n = 181). GLC-mass spectrometric techniques established analysis specificity.