A specific and sensitive isocratic method for the measurement of a new anticonvulsant, (S)-3-(aminomethyl)-5-methylhexanoic acid, in rat plasma and milk is described. Following deproteinization, the compound and internal standard [1-(aminomethyl)cycloheptaneacetic acid] were derivatized utilizing 2,4,6-trinitrobenzene sulfonic acid and extracted with cyclohexane. Analytes were resolved on a 5 microns Spherisorb ODSII column (250 mm x 4.6 mm) using a mobile phase of 57% acetonitrile in 0.1 M ammonium acetate, pH 4.0. Absorbance was monitored at 350 nm. Limit of quantitation was 1.00 microgram/ml for a 100-microliters aliquot of plasma or milk.