Safrole, a naturally occurring product derived from spices and herbs, has been shown to be associated with the development of hepatocellular carcinoma in rodents. Safrole 2',3'-oxide (SFO), an electrophilic metabolite of safrole, was shown to react with DNA bases to form detectable DNA adducts in vitro, but not detected in vivo. Therefore, the objective of this study was to investigate the formation of N7-(3-benzo[1,3]dioxol-5-yl-2-hydroxypropyl)guanine (N7γ-SFO-Gua) resulting from the reaction of SFO with the most nucleophilic site of guanine in vitro and in vivo with a newly developed isotope-dilution high performance liquid chromatography electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS) method. N7γ-SFO-Gua and [ 15N 5]-N7-(3-benzo[1,3]dioxol-5-yl-2-hydroxypropyl)guanine ([ 15N 5]-N7γ-SFO-Gua) were first synthesized, purified, and characterized. The HPLC-ESI-MS/MS method was developed to measure N7γ-SFO-Gua in calf thymus DNA treated with 60μmol of SFO for 72h and in urine samples of mice treated with a single dose of SFO (30mg/kg body weight, intraperitoneally). In calf thymus DNA, the level of N7γ-SFO-Gua was 2670 adducts per 10 6nucleotides. In urine of SFO-treated mice, the levels of N7γ-SFO-Gua were 1.02±0.14ng/mg creatinine (n=4) on day 1, 0.73±0.68ng/mg creatinine (n=4) on day 2, and below the limit of quantitation on day 3. These results suggest that SFO can cause in vivo formation of N7γ-SFO-Gua, which may then be rapidly depurinated from the DNA backbone and excreted through urine.