A procedure is described for the determination of nitrofurantoin and its toxic metabolite in urine from patients with urinary infection using supercritical fluid extraction (SFE) and liquid chromatography. The standard solution of toxic metabolite (radical anion) was obtained by electrochemical reduction of nitrofurantoin in an aprotic medium and chemical reoxidation with oxygen. In our initial SFE studies to find the adequate extraction parameters, drug solutions were impregnated onto filter paper. Quantitative extractions were achieved when the experiments were carried out under 2500 psi of pressure at a temperature of 80 degrees C (oven and restrictor) after 20 min of static extraction and 5 min of dynamic extraction. The modifier used was acetonitrile (2.0 ml in a 10 ml extraction column). Nitrofurantoin and its toxic metabolite were detected in urine samples. Both compounds were quantified in the extracts by high performance liquid chromatography (HPLC) with detection at 310 nm. The calibration graph of these compounds in acetonitrile was linear between 10.9 and 378.0 muM (R=0.9995) for nitrofurantoin and between 3.0x10(-3) and 21.0 muM (R=0.9992) for the metabolite. The detection limits (LOD) were 12.1 and 0.9 muM, respectively. The drug was administered to two patients during 7 days, and all the urine eliminated between 1 day before and 2 days after administration was analyzed. One patient consumed the drug in the form of microcrystals and the other as macrocrystals.