The measurements of the intravascular HbO2 saturation in tumor capillaries using the cryophotometric micromethod reveal that very low HbO2 saturation values predominate in malignant tumors. Under normoxic conditions only 8% of the measured values exceed 50% saturation. 53% of the intracapillary HbO2 saturation values are in the range of 0-10% HbO2 saturation. Great regional differences are seldom and can be found only in areas where a sufficient vascularization still exists. Taking into account the data of morphometric analysis of tumor vascularization and the parameters of respiratory gas exchange, the measured frequency distribution of HbO2 saturation values in tumor capillaries is simulated by means of different three-dimensional capillary structures; On the basis of these microcirculatory units, tissue pO2 values are computed in the intercapillary regions. As a result of these calculations it can be stated that the computed data derived from intracapillary HbO2 saturations are in sufficient agreement with pO2 values measured polarographically using gold- microelectrodes.