The effect of paclitaxel on the accumulation of gemcitabine, diflourodeoxyuridine (dFdU) and the phosphorylated metabolites of gemcitabine. The cells were treated with vehicle-control or paclitaxel at the observed IC-50 value for 24 hours followed by gemcitabine at the observed IC-50 value for 24 hours. The cell medium was collected and the cells manually harvested by a cell scraper; the medium and cells were stored at -80°C until analysis. Gemcitabine and its metabolites were below the limits of quantitation in two of the three cell lines. The accumulation of phosphorylated metabolites within the cells was measurable in (a) H520 cells and the accumulation of gemcitabine and dFdU in the medium were measurable in (b) H520 cells. The diphosphate levels were significantly lower in paclitaxel treated cells compared to vehicle-control treated cells (P < 0.05). Gemcitabine levels were significantly higher in paclitaxel treated cells compared to vehicle-control treated cells (P < 0.001).