A simple, sensitive and selective liquid chromatography tandem mass spectrometry (LC-MS/MS) method was developed and validated for the analysis of venlafaxine (VX) and its major active metabolite O-desmethyl venlafaxine (ODV) in rat plasma using carbamazepine as an internal standard (IS). The analytes from the biological matrix were extracted by liquid-liquid extraction using tert-butyl methyl ether (TBME). The HPLC separation of the analytes was performed on a water symmetry C18 (150 x 4.6 mm i.d., 5 microm) column, using a 10 mM ammonium formate:methanol (20:80 v/v) as mobile phase. The calibration curve concentration range was 10.10 to 8000.00 ng/mL for VX and ODV with coefficient of determination above 0.9970. The lower limit of quantification (LLOQ) of VX and ODV were 3.35 and 3.86 ng/mL, respectively. The intra- and inter-day coefficients of variation were within 15%.