A selective and sensitive LC/MS/MS assay was developed for the quantification of d 2 -nicotine and d 2 -cotinine in plasma of current and past smokers administered d 2 -nicotine. After solid phase extraction and liquid liquid extraction, HPLC separation was achieved on a capillary hydrophilic interaction chromatography phase column. The analytes were monitored by tandem mass spectrometry with electrospray positive ionization. Linear calibration curves were generated for d 2 -nicotine (0.03 to 6.0 ng/ml plasma) and d 2 -cotinine (0.15 to 25 ng/ml plasma). The lower limits of quantitation were 0.15 ng/ml and 0.25 ng/ml for d 2 -nicotine and d 2 -cotinine, respectively. The coefficient of variation was 3.7% for d 2 -nicotine and 2.5% for d 2 -cotinine. The method was applied to two ongoing studies of d 2 -nicotine metabolism in prior and current smokers. Preliminary analysis of a subset of subjects from these studies detected a significantly lower rate of nicotine conversion to cotinine by past smokers compared to current smokers.