An ion-pairing reversed-phase HPLC assay employing amperometric detection has been developed for the determination of free minoxidil (MNX) in human serum. The drug is isolated from the serum and concentrated by solid phase extraction with a disposable cartridge column containing ethylsilane (C2) bonded phase packing material. The average absolute recovery from serum was 85%. The HPLC separation is performed on a Sperisorb-C(8) column, using n-octanesulphonic acid as the pairing ion. The method exhibits linear behaviour from 0.3 to 100 ng/ml for spiked serum samples. The average daily relative standard deviations of replicate samples at the 0.75 and 2.0 ng/ml levels were 9.6 and 6.4%, respectively. Utilizing a 1 ml sample the limit of detection (S/N > or = 3) was 0.3 ng/ml.