A stereospecific high-performance liquid chromatography (HPLC) method has been developed for the analysis of the underived enantiomers of citalopram (CIT) and its N-demethylated metabolites in plasma. Using fluorescence detection, the limit of quantification for each enantiomer is 3 ng/ml. CIT N-oxide and the CIT propionic acid derivative are not extracted by the procedure used. Inter- and intraday validations of the method using reverse-phase mode HPLC on separate acetylated beta-cyclobond columns showed the sensitivity of this assay to be suitable for pharmacokinetic studies of the enantiomers of these compounds. Plasma levels of the enantiomers and the demethylated metabolites of CIT have been determined during routine therapeutic drug monitoring (TDM) in 29 depressive patients treated with varying dosages (20-80 mg/day) of CIT. Concentrations of S-(+)-CIT, which is considered the most potent selective serotonin reuptake inhibitors (SSRI) of the CIT and desmethylcitalopram (DCIT) enantiomers, varied between 24-49% (mean +/- sd, 35% +/- 5%) of the concentrations of total CIT. There were highly significant correlations between S-(+)-CIT and R-(-)-CIT levels (r = 0.866; p < 0.0001) and between S-(+)-DCIT and R-(-)-CIT (r = 0.932; p < 0.0001). The co-medications seemed to have little influence on enantiomer ratios. These results suggest the need for studies on the relationships between clinical response and plasma levels of CIT enantiomers.