Summary. The development, optimization, and validation of a new high-performance liquid chromatography ultraviolet (HPLC-UV) method is presented for determining 2-(3-chlorophenyloamino)-5-(2,4-dihydroxy-phenyl)-1,3,4-thiadiazole (ClABT) in biological samples of rat plasma and brain tissue. ClABT was extracted directly from a plasma supernatant fraction following protein precipitation with acetonitrile and high speed centrifugation. Reverse phase HPLC separation was performed using an ODS-2 Hypersil column with the mobile phase consisting of 0.05 M triethylammonium phosphate buffer solution in acetonitrile and methanol (120:280:600, v/v/v), at room temperature, 1.2 mL min−1 flow rate, and UV-diode-array detection (DAD), at 335 nm. A linear response was obtained between 12.5 and 2000 ngmL−1 at analytically acceptable levels of precision (intra/inter-day) and accuracy. Mean recoveries ranged from 92.7% to 107.9%. It was concluded that the method was specific and precise and thus suitable for quantitative analysis in clinical pharmacokinetic studies of ClABT.