Three principles for the use of HPLC with spectrophotometric detection to determine the concentration of furosemide and of 4-chloro-5-sulfamoyl anthranilic acid (CSA) were studied. A reversed phase microbondapack C18 column was used for the separation of either unchanged furosemide (I), CSA (II) or the diazo product of CSA (III). The sensitivity of the methods for the determination of furosemide added to serum in vitro were for I 0.05, for II 0.020 and for III 0.01 microgram/ml. The methods I and II were used to study the excretion pattern in the urine of furosemide and CSA after intravenous injection of 80 mg to 10 normal subjects. An average of 48.1 +/- 9.8% (S.D.) of the furosemide was excreted during the first hour as unchanged furosemide. Simultaneously 1.9 +/- 0.8% of the dose was excreted as CSA. Only very low concentrations of CSA were found in serum and that metabolite apparently was excreted with a higher renal clearance than furosemide during the period 30-60 min. after the administration (30.3 +/- 145 ml/min. for CSA and 88 +/- 23 ml/min. for furosemide). During the 24 hour period following the administration 52.6 +/- 8.1 mg was excreted as unchanged furosemide and 11.4 +/- 5.2 mg as a glucuronidated product.