To monitor the levels of caffeic acid in rat blood, an on-line microdialysis system coupled with liquid chromatography was developed. The microdialysis probe was inserted into the jugular vein/right atrium of male Sprague-Dawley rats. Caffeic acid (100 mg/kg, i.v.) was then administered via the femoral vein. Dialysates were automatically injected onto a liquid chromatographic system via an on-line injector. Samples were eluted with a mobile phase containing methanol-100 mM monosodium phosphoric acid (35:65, v/v, pH 2.5). The UV detector wavelength was set at 320 nm. The detection limit of caffeic acid was 20 ng/ml. The in vivo recoveries of the microdialysis probe for caffeic acid at 0.5 and 1 microg/ml were 48.34+/-2.68 and 47.64+/-3.43%, respectively (n=6). Intra- and inter-assay accuracy and precision of the analyses were < or = 10% in the range of 0.05 to 10 microg/ml. Pharmacokinetics analysis of results obtained using such a microdialysis-chromatographic method indicated that unbound caffeic acid in the rat fitted best to a biexponential decay model.