Abstract: The plasma concentration of melatonin in rabbits are measured by GC/MS with selected ion (m/z 173) and caffeine(m/z 194) as internal standard to determine the pharmacokinetic of melatonin. The concentrtion-time profile of melatonin is abtained after melatonin 30 mg·kg -1 administered for a two-compartment open model in rabbits. The pharmacokinetic parameters are t 1/2α=0.31 h, t 1/2β=8.58 h, t 1/2Ka=0.34 h, T max=0.63±0.47 h, C max=322.23±255.40 ng/mL, K 12=0.81 h -1, K 21=2.80 h -1, K 10=2.10 h -1, AUC=697.49(ng/mL)*h,CL(s)=0.097±0.054(mg/kg)/h(ng/mL). The method is stable, sensitive and accurate. It is a useful method for the determination of pharmacokinetics of melatonin which is important for clinic.