19F nuclear magnetic resonance was used for a noninvasive and quantitative study of flucytosine (FC; 5-fluorocytosine) metabolism in two strains of Candida albicans and one strain of Candida tropwcalis with various susceptibilities to FC. Three intracellular fluorinated metabolites were detected in the highly susceptible strain, F-nucleotides (Fnt), F-nucleosides, and 5-fluorouracil (5FU). Fnt were partially converted into 5FU when the spectra of the yeasts were recorded at 37°C without perfusion, but the intensities of the signals were not modified at 4 or 37°C if the cells were perfused. In the acid extract, the Fnt signal was resolved into three distinct peaks; none of them was attributable to 5-fluoro-2'-deoxyuridine-5'-monophosphate. The same signals were detected in the partially resistant strain, but only 5FU was observed in the highly resistant strain; the resistance of the latter strain therefore was primarily due to a defect in UMP pyrophosphorylase. At the end of the incubation period, only FC and released 5FU were present in the culture media. The concentration of the intracellular fluorinated metabolites was increased if the strain was susceptible to FC. The total amount of metabolized FC was very similar for the highly susceptible and the partially resistant strains, but the percentage of Fnt was much higher in the former (38%) than in the latter (8%); the mild resistance of the partially resistant strain therefore was attributed to the decreased activity of UMP pyrophosphorylase.Flucytosine (FC; 5-fluorocytosine) has gained an important place in the systemic treatment of deep-seated fungal infections in humans, in particular against candidiasis, cryptococcosis, and chromomycosis (16). Investigation of the intrafungal pathway of the drug is an important key to the understanding of the mechanisms of drug action and drug resistance. Previous studies of the FC mode of action have involved invasive techniques requiring extraction procedures and, most often, the use of labeled drugs (5,15,19,20).We used '9F nuclear magnetic resonance (NMR) to study the mode of action of FC. This noninvasive technique allows a direct analysis of heterogeneous biological samples without prior extraction and does not require a labeled drug, as the observed probe, the '9F nucleus, is an inherent part of the parent drug and its metabolites. Moreover, this technique permits simultaneous identification and quantification of all the fluorinated metabolites present in the analyzed sample (1). We have already applied this methodology to metabolism studies of antineoplastic fluoropyrimidines, 5-fluorouracil (5FU), and 5'-deoxy-5-fluorouridine in human biofluids and cultured tumor cells (12, 13;