A reversed-phase ion-pair high-performance liquid chromatography method for the determination of acyclovir and its metabolite 9-carboxymethoxymethylguanine is described. The sample are purified by reversed-phase solid-phase extraction. The components are separated on a C18 column with a mobile phase containing 18% acetonitrile, 5 mM dodecyl sulphate and 30 mM phosphate buffer, pH 2.1, and measured by fluorescence detection using an excitation wavelength of 285 nm and an emission wavelength of 380 nm. Detection limits are 0.12 microM (plasma) and 0.60 microM (urine) for acyclovir, and 0.26 microM (plasma) and 1.3 microM (urine) for metabolite. Correlation coefficients that were better than 0.998 were obtained normally. This analytical method, which enables simultaneous measurement of parent compound and metabolite, has been used in kinetics studies and for therapeutic drug monitoring in different patient groups with variable degrees of renal dysfunction.