A sensitive method for monitoring serum or plasma labetalol (2-hydroxy- 5− [1− hydroxy- 2− (1− methyl- 3− phenylpropylamino) ethyl] benzamide hydrochloride) is described. The method is designed to study single dose kinetics and assess predose labetalol levels during chronic treatment. The applicable range of the assay is 10 to 400 ng/ml using 1 ml of plasma or serum. The method uses reverse-phase high pressure liquid chromatography (HPLC), a mobile phase of acetonitrile/pH 3.0 phosphate, and ultraviolet detection at 207 nm. The percentage recovery of labetalol and internal standard (propericiazine) (3-cyano-10-[3–4-hydroxypiperidino) propyl] phenothiazine from serum was 95.6% and 75.5%, respectively. The precision of the method at labetalol serum concentrations of 400 and 50 ng/ml was indicated by a coefficient of variation of 4.3% and 6.9%, respectively. An approximate fourfold variation in plasma labetalol concentration, with plasma levels ranging from 20 to 93 ng/ml, was seen 10 to 11 hours after a dose in a group of patients taking 200 mg twice daily.