Abstract: A more sensitive procedure was developed to phenotype cytochrome P450 2D6 metabolism by a less toxic probe drug—dextromethorphan. The sensitivity of the determination of dextromethorphan in urine samples was up to 1 ng·ml^-1. This method was employed to phenotype three groups of subjects. The results showed that there was a difference between extensive metabolizers which were genotyped as homozygotes and heterozygotes. Although the difference was not as big as the difference with poor metabolizers which were genotyped as homozygous mutants. It suggests that “gene copy” is a true factor in the expression of CYP2D6 genes.