The high-performance liquid-chromatographic method described here for the quantitative analysis for nitrofurantoin in urine and plasma involves direct analysis of urine samples and analysis of plasma samples after protein precipitation by methanol. The assay, which requires only 0.2 ml of biological fluid and shows a linear relationship in the range 0.02 to 200 mg/liter, can be performed in 9 min and is reproducible (CV less than 2%). Results for nitrofurantoin so obtained correlate well with those obtained by the Hyamine 10-X spectrophotometric method (CV = 2%), but the present method is more sensitive. With no modification, the present procedure can also be used for nitrofurazone. The sensitivity, accuracy, and convenience of the method make it suitable for clinical monitoring and pharmacokinetic/bioavailability studies with 5-nitrofuran derivatives.