A simple and sensitive high performance liquid chromatographic method was developed for determination of ciprofloxacin (Cip) in 0.1 ml of rabbit and human serum and CSF. Separation of Cip and pipemidic acid (internal standard) was achieved on Nucleosil C18 (4.6 mm x 250 mm) using fluorescence detection with lambda exc 274 nm and lambda emi 418 nm. The mobile phase was composed of acetonitrile and phosphate buffer 10 mmol.L-1 (pH 2.7) containing tetrabutylammonium hydrogen sulfate 5 mmol.L-1 (18:82, vol:vol) and the flow rate was set at 1.0 ml.min-1. The retention times were 5.9 min for Cip and 4.0 min for pipemidic acid. The inter-day coefficient of variation was < 6.97% (n = 5) and the intra-day coefficient of variation was < 3.33% (n = 5). The limits of detection were 3 ng.ml-1 serum and 5 ng.ml-1 CSF, (r > or = 0.9996). Application of this method was demonstrated with simultaneous measurements of concentration-time profiles of Cip in rabbit serum and CSF during iv infusions at constant rates of 0.33, 1.0, and 2.5 mg.kg-1.h-1.