In adult liver, CYP3A4 plays an important role in the metabolism of a wide range of endogenous and exogenous compounds. To investigate whether there is a single nucleotide polymorphism(SNP) of CYP3A4 intron 2 in the liver and its effects on the mRNA expression and enzymatic activity of CYP3A4, genomic DNA was extracted from 96 liver tissue samples obtained from patients who had undergone liver surgery. An SNP of CYP3A4 intron 2 was identified by polymerase chain reaction(PCR)-single-strand confirmation polymorphism and DNA sequencing. The mRNA expression of CYP3A4 was determined by the fluorescence quantitative PCR technique. The enzymatic activity of CYP3A4 was measured using erythromycin and testosterone as probe substrates. Twelve patients were found to have the SNP/T4127 G CYP3A4 within intron 2. The mRNA levels of CYP3A4 in wild-type and SNP/T4127 G samples were 2.62±1.09 and 2.79±1.63, respectively(P>0.05). Erythromycin N-demethylase activity in wild-type and SNP/T4127 G samples were 121.2±32.8 and 124.7±61.6 nmol·mg-1·min-1, respectively(P>0.05). The activity of testosterone 6β-hydroxylase was significantly different between wild-type(648±173 pmol·mg-1·min-1) and SNP/T4127 G samples(540±196 pmol·mg-1·min-1; P<0.05). In conclusion, the SNP/T4127 G of CYP3A4 intron 2 exists in the liver. This SNP does not affect the mRNA expression of CYP3A4 but significantly decreases the hepatic microsomal testosterone 6β-hydroxylase activity of CYP3A4. Furthermore, this study indicates that the appropriate selection of probe substrates is very important in studying the relationship between the genotype and phenotype of CYP3A4. In investigate whether there is a single nucleotide polymorphism (SNP) of CYP3A4 intron 2 in the liver and its effects on the mRNA expression and enzymatic activity of CYP3A4, genomic DNA was extracted from 96 liver tissue samples obtained from patients who had undergone liver surgery. An SNP of CYP3A4 intron 2 was identified by polymerase chain reaction (PCR) -single-strand confirmation polymorphism and DNA sequencing. The mRNA expression of CYP3A4 was determined by the fluorescence quantitative PCR technique. The enzymatic activity of CYP3A4 was measured using erythromycin and testosterone as probe substrates. Twelve patients were found to have the SNP / T4127 G CYP3A4 withintron 2. The mRNA levels of CYP3A4 in wild- type and SNP / T4127 G samples were 2.62 ± 1.09 and 2.79 ± 1.63, respectively (P> 0.05). Erythromycin N-demethylase activity in wild-type and SNP / T4127 G sample s were 121.2 ± 32.8 and 124.7 ± 61.6 nm · mg -1 · min -1, respectively (P> 0.05). The activity of testosterone 6β-hydroxylase was significantly different between wild-type (648 ± 173 pmol · mg-1 · In SNP / T4127 G of CYP3A4 intron 2 exists in the liver. This SNP does not affect the SNP / T4127 G samples (540 ± 196 pmol · mg-1 · min-1; P <0.05) affect the mRNA expression of CYP3A4 but significantly reduces the hepatic microsomal testosterone 6β-hydroxylase activity of CYP3A4. Furthermore, this study indicates that the appropriate selection of probe substrates is very important in studying the relationship between the genotype and phenotype of CYP3A4.