A method for measuring plasma prazosin concentrations is reported. This method, which involves high pressure liquid chromatography (HPLC) combined with fluorescence detection, has a sensitivity 20 times that of previous conventional fluorimetric techniques and twice that of other HPLC methods, and can be used to study the pharmacokinetics of prazosin at very low doses.Plasma prazosin concentrations were measured in 5 normal volunteers for 24 hours after single oral doses of 0.5 and 1.5mg of prazosin. On average, the lag time was 0.650 and 0.448 hours, and the elimination half-life 2.3 and 2.5 hours, for the 0.5 and 1.5mg doses, respectively. Peak prazosin concentrations occurred between 1.1 and 3.6 hours after dosing, and after the 1.5mg doses were associated with symptoms of faintness.