Radiation response of some tumors is dependent on the oxygenation of the tumor tissue. To improve tissue oxygenation, attempts to increase a reduced hemoglobin concentration or to shift the dissociation curve of hemoglobin have been made. The aim of this paper is to estimate the influence of such measures on the volume of radiobiologically hypoxic tissue by means of a mathematical model. In addition, the influence of blood velocity and metabolic status of tumor tissues on tissue oxygenation is evaluated. The calculations show a strong influence on hypoxic tissue volume over a modest range of variation of physiological parameters, especially when interactions of several parameters may occur.