A simple high-performance liquid chromatographic method has been developed for the measurement of atenolol in plasma, breast milk, and urine. The sample (250 microliters) is Vortex-mixed for 30 s with approximately 50 mg sodium chloride, 10 mol/L sodium hydroxide solution (50 microliters), internal standard solution (aqueous benzimidazole, 1.69 mumol/L) (50 microliters) and methyl-tert-butyl ether containing 10% (vol/vol) heptafluorobutanol (200 microliters). After centrifugation (9950 X g, 2 min), a portion (100 microliters) of the resulting extract is analysed on a microparticulate (5 micron) silica column using methanol containing 1 mmol/L d-10-camphorsulphonic acid monohydrate as the mobile phase, and the column effluent is monitored by fluorescence detection using an excitation wavelength of 195 nm. A specimen, together with a quality control sample, can be analysed in duplicate within 30 min. The limit of accurate measurement of the assay is 20 micrograms/L, no endogenous sources of interference have been observed and interference from other drugs is minimal.