A method for determination of carnitine, 4-(N,N,N-trimethylammonio)butanoate (butyrobetaine), and 2-(N,N,N-trimethylammonio)acetate (betaine) is described. These omega-trimethylammonio carboxylates and the chemically analogous internal standards 4-(N,N-dimethyl-N-propylammonio)-3-hydroxybutanoate or 6-(N,N,N-trimethylammonio)hexanoate were derivatized by reaction with 4'-bromophenacyl triflate in the presence of N,N-diisopropylethylamine. The trialkylammonio carboxylate 4'-bromophenacyl ester derivatives were separated from other sample constituents by reversed-phase ion-pair high-performance liquid chromatography with spectrophotometric detection at 254 nm. Standard curves were linear over a sample concentration range of 10-100 nmol/ml. Quantities of 2.5 nmol of omega-trialkylammonio acid derivatives injected into the chromatograph were detected with signal-to-noise ratios greater than 50.