Abstract: A three-porous-medium model for transvascular exchange and extravascular transport of fluid and macromolecules in a spherical solid tumor is developed. The microvasculature, lymphatics, and tissue space are each treated as a porous medium with the flow of blood, lymph, and interstitial fluid obeying Darcy's law and Starling's assumption. In this part, the role of int erstitial pressure and fluid convection are st udied. The analytical solutions are obtained for the isolated tumor and the normal-tissue-surrounded tumor respectively. The calculated interstitial pressure profile are consistent with the experimental observation that the elevated interstitial pressure is a major barrier in the penetration of macromolecular drug into tumors. The factors which may reduce the interstitial pressure are analyzed in details.