The stability of flucytosine in an oral liquid prepared from capsules was studied. Flucytosine 10-mg/ml oral liquid was prepared from 500-mg capsules (with distilled water as the diluent) and transferred to 10 glass and 10 plastic amber bottles. Five of each type of bottle were stored at 4 degrees C and five of each at 25 degrees C. Samples were taken at 0, 7, 14, 28, 42, 56, 70, and 91 days, and flucytosine concentration was measured by stability-indicating high-performance liquid chromatography. Flucytosine was stable in the liquid formulation for at least 70 days under all the study conditions. The physical appearance of the liquid stored at 4 degrees C did not change during the study period. Flocculation was observed both in glass and in plastic bottles stored at 25 degrees C on day 14 and thereafter. Flucytosine 10 mg/mL in an extemporaneously prepared oral liquid was stable for 70 days in glass or plastic prescription bottles kept at 4 or 25 degrees C.