We describe a "high-performance" liquid-chromatographic assay for measuring ritalinic acid in 300 muL of serum. The procedure includes adsorption of ritalinic acid and internal standard (alpha,alpha-dimethyl-beta-emthylsuccinimide) from serum onto charcoal, from which both compounds are eluted with methanol. The methanol extract is evaporated, the residue dissolved in a 93/7 (by volume) mixture of potassium phosphate buffer (20 mmol/L, pH 3.8) and acetonitrile, and an aliquot of this solution is chromatographed. We used a mu-Bondapak C-18 column with the above-mentioned buffer system as mobile phase, a flow rate of 2.0 mL/min, and a column temperature of 40 degrees C. Ritalinic acid and internal standard are detected by their absorbance at 192 nm and quantitated by measuring peak heights. The procedure allows for the reliable analysis for ritalinic acid in serum at concentrations greater than 50 microgram/L. Analytical recoveries were greater than 73% and the method affords good day-to-day precision (CV, less than 12%).