Intratumoral infusion has a potential for improving distribution of drugs. To optimize the infusion, we developed a novel technique to quantify the distribution volume of color molecules (V_d) in solid tumors. Evans blue-labeled albumin was infused locally with the use of a needle into a rat fibrosarcoma ex vivo under different pressures. After the infusion, tumor tissues were sectioned serially into thin slices. The blue area in each slice was quantified with the use of the newly developed technique. The V_d was calculated based on the blue area and the slice thickness. Our data showed that infusion pressure and volume (V_i) had significant effects on V_d. The median of V_d/V_i decreased from 2.99 to 1.79 when infusion pressure was increased from 50 to 163 cmH₂O, presumably due to retardation of convective transport. In addition, the coefficient of variation in V_d/V_i was increased from 0.13 at 50 cmH₂O to 0.64 at 163 cmH₂O. The dependence of V_d/V_i and its variation on infusion pressure suggests that 1) infusion-induced tissue deformation is unpredictable and 2) both the unpredictability and the interstitial retardation of convective transport increase with infusion pressure.