A method for the determination of choline in human plasma is described, involving rapid purification of plasma samples and analysis by high-performance liquid chromatography using an on-column enzyme reactor with electrochemical detection. The linearity of the method was tested at choline levels from 3.5 to 28.6 microM in plasma. The recovery was 86% and was independent of the analyte concentration. The inter-assay precision (as coefficient of variation) and accuracy (as the deviation of the concentration found from the theoretical value) were always below 12% in the whole concentration range. The method was applied to the determination of plasma choline levels in eight healthy volunteers after intramuscular administration of L-alpha-glycerophosphorylcholine (1 g) or a placebo. Mean plasma choline levels in the placebo group ranged from 10.6 to 12.0 microM. After drug administration, the plasma choline level reached 35.1 microM in 30 min, then decreased gradually. Plasma choline levels became comparable in the treated and placebo groups 6-8 h after administration.