A sensitive HPLC method for the determination of fenofibric acid (FA), the active form of fenofibrate in serum is described. FA from human serum samples was isolated by an easy one-step extraction procedure with a mixture of n-hexane and ethylacetate (90:10, v/v). The recovery was 84.8% of the total Fa in serum. The compound was separated isocratically on a reversed phase with acetonitrile and 0.02 M phosphoric acid (55:45, v/v) at a flow-rate of 1.0 ml/min. Absorbance at 287 nm was recorded for quantification. Validation presents a detection limit of 0.03 microgram/ml and a quantification limit of 0.1 microgram/ml (relative standard deviation at 0.1 microgram/ml = 7.1%). For an extensive validation of this method we determined the serum levels of FA in one young male volunteer and examined the pharmacokinetics of standard, mikronized and slow release formulation of fenofibrate after oral intake. This method is a rapid and reliable tool for quantitative determination of fenofibric acid in pharmacokinetic investigations.