A sensitive and selective high-performance liquid chromatographic assay for the quantiﬁcation of ketanserin and ketanserinol in human plasma was developed and validated. The procedure involves extraction of ketanserin and ketanserinol from plasma using an Extrelut NT-1 solid-phase extraction column. The chromatograph was equipped with a Hypersil BDS column (100 × 4.5 mm, 3 µm particle size). Separation was performed with a mixture of acetate buffer 0.01 M, pH 4.9–methanol–acetonitrile (52:40:8, v/v/v). Detection was performed with ﬂuorescence detection (λex = 332 nm and λem = 410 nm). Calibration curves were linear (r2 = 0.999) in the range 0–400 ng/mL for both ketanserin and ketanserinol. The repeatability coefﬁcient for ketanserin and ketanserinol was 3.1 and 3.0%, respectively. The reproducibility coefﬁcient for ketanserin and ketanserinol was 10.5 and 9.1%, respectively. The limit of quantiﬁcation for both ketanserin and ketanserinol was 2.0 ng/mL. The mean recovery yield for both ketanserin and ketanserinol was 60%. In an 8 h work day approximately 60 samples, including calibration and reference standards, could be processed. Copyright © 2003 John Wiley & Sons, Ltd.