Sensitive liquid chromatography (LC)/electrospray ionization (ESI) tandem mass spectrometry (MS) can be used to analyze the bile acid composition of rat serum. This method can analyze eight common bile acids and their glycine and taurine conjugates in 100 d rodent serum by gradient elution on a reversed-phase column using a mixture of 20 mM arnmoniurn acetate buffer (pH 8.0), acetonitrile and methanol as a mobile phase. Selected reaction monitoring analysis under negative ion detection mode allowed the achievement of a high sensitive assay with a simple solid phase extraction using an ODS cartridge column. We used this method to investigate the effect of a one-day fast on the concentration and cornposition of serum bile acids in rats. The results suggested that the method described here is useful for the dynamic analysis of serum bile acids in rats. (c) 2005 Elsevier B.V. All rights reserved.