Using capillary zone electrophoresis with a phosphate buffer at pH 2.5 containing 50 mM heptakis-(2,3,6-tri-O-methyl)-beta-CD as chiral selector, the separation of the enantiomers of the main metabolite of debrisoquine (DEB), 4-hydroxydebrisoquine (4-OHDEB), is reported. For extraction of underivatized urinary DEB, S-4-OHDEB and R-4-OHDEB, a procedure using disposable cartridges containing a polystyrene-based polymer was developed. A few nL of the extracts were analyzed in a 60 cm fused-silica capillary of 50 microns ID and solute detection was effected at 195 nm. For all three compounds, a mean (n = 5) recovery of about 73% and a detection limit of about 150 ng/mL were noted. Data obtained with urines that were received for routine phenotyping with DEB and mephenytoin confirmed the almost exclusive formation of S-4-OHDEB. Under the described conditions, no R-4-OHDEB could be detected. With these data and those obtained employing no chiral selector in the buffer, differentiation between extensive metabolizer phenotypes (EM) and poor metabolizer phenotypes (PM) for DEB was unambiguously possible by the presence of a significant peak and no (or minor) peak for 4-OHDEB, respectively. Data obtained for ten EM subjects and five PM subjects were found to agree with those generated by the routine assay based on gas chromatography. The capillary electrophoretic assays described are simple, reproducible (relative standard deviation of peak area ratios < 3%), require no sample derivatization, consume no halogenated organic solvents, and operate with inexpensive separation columns as well as small amounts of chemicals.