OBJECTIVE:To establish an LC-MS method for the determination of the ritodrine content in human plasma.METHODS:The determination was performed on Agilent Eclipse Plus C18 column with the column temperature kept at 40 ℃.The mobile phase consisted of methanol-water(95∶5,containing 0.02 mol?L-1 ammonium formate)with a flow rate of 0.5 mL?min-1.The extraction agent was acetic ether-dichloromethane(4∶1).A triple quad mass spectrometer system equipped with an electro spray ionization ion-trap source was used as the detector and operated in the positive ion model.Selected reaction monitoring(SRM)was employed to detect ritodrine(m/z 288.2→121.1)and the internal standard dobutamine(m/z 302.1→107.0).RESULTS:The linear range of ritodrine was 0.25～100 ng?mL-1(r=0.999 1),with the lower limit of quantification at 0.25 ng?mL-1.Both the intra-day RSD and inter-day RSD were less than 15%.The methodological recovery was 98.00%～103.44%.CONCLUSION:The method is sensitive,accurate,reproducible,specific and suitable for the pharmaceutical study of ritodrine.