A rapid, simple, sensitive LC-MS/MS method involving a least pretreatment process has been proposed for the quantitative assessment of venlafaxine (VEN) and O-desmethyl venlafaxine (ODV) using cetirizine as an internal standard. The method was validated over the range of 1.03 ng/mL to 453.50 ng/mL (venlafaxine) and 1.32 ng/mL to 585.21 ng/mL (O-desmethyl venlafaxine). The lowest limit of quantification for venlafaxine and O-desmethyl venlafaxine was found to be 1.03 ng/mL and 1.32 ng/mL, respectively. The solid phase extraction procedure provided reliable and reproducible recoveries of the drug as well as its active metabolite with no interference at their retention time. The recovery for all analyzed drugs was found to be in the range of 72.55% to 74.75%. The result indicates that the developed procedure could be considered suitable for carry out simultaneous preclinical pharmacokinetics studies for VEN and ODV.