Apigenin (4',5,7-trihydroxyflavone, AP) belongs to a less-toxic and non-mutagenic flavone subclass of flavonoids, the biotransformation and metabolism of which have been little studied until now. Therefore, this study is focussed on the determination of AP in free form. AP was administered to rats via the i.p. route (25 mg kg(-1)) and then the blood was collected at 10, 15, 30 and 45 min after injection. Methanol was used for rat plasma deproteinization. The HPLC assay (mobile phase, 2% formic acid-acetonitrile-methanol, 40:35:25, v/v; flow-rate, 1 ml min(-1); UV detection at 349 nm) for AP determination was validated and used for the quantification of AP in rat plasma. The unknown concentration was calculated from the equation obtained by the least-squares regression analysis (y = 0.521x + 1.130, r2 = 0.998). The highest concentration of AP in plasma was found to be 30 min after injection. The concentration profile of AP obtained here may contribute to until known results about AP metabolism. They could be applied to other studies of AP or related flavonoids because of favourable effects on human health.