A simple and convenient method using commercially available derivatization reagents is described for the measurement of betaine and N,N-dimethylglycine (DMG) in blood and urine. Precolumn derivatization of plasma or urine is performed directly in acetonitrile without extraction with p-bromophenacyl bromide and crown ether as catalyst. The p-bromophenacyl ester derivatives are then separated by high-performance liquid chromatography, using an isocratic system of acetonitrile and water containing choline. Effluent was monitored at 254 nm. The limit of detection was 5 micromol/L for betaine and 2 micromol/L for DMG. Analytical recovery was >97% for both analytes. Total and within-day CVs were 2.0-4.4% and 0.9-2.2% for DMG. For betaine, the total and within-day CVs were 1.3-5.3% and 0.4-3.8%, respectively. The method is precise and cost-effective and has been used successfully to determine the concentrations of DMG and betaine in human plasma and urine.