Deoxycytidine (CdR) analogs are increasingly popular as chemotherapeutic agents and their effectiveness can be linked to the direct competition with active forms of endogenous CdR. A tandem mass spectrometric assay was developed to determine the plasma concentrations of CdR. Plasma extracts were prepared by protein precipitation and an ethyl acetate/water back extraction, and then separated chromatographically. Detection parameters were optimized for multi-reaction monitoring (MRM) tandem mass spectrometry and assay efficiency was improved using 15N3 CdR as an isotopic internal standard. Preliminary results from a gemcitabine trial are shown which indicate that CdR concentrations increase systemically during infusion, from about 5 nM to 78 nM after hepatic artery infusion and to 102 nM after systemic infusion for 24 hours. The developed assay demonstrated good sensitivity and selectivity for CdR.