Blood flow of human mammary carcinomas heterotransplanted into immune- deficient rnu/rnu-rats is low. It decreases further with increasing tumor wet weight, the mean value being comparable to that of mammary carcinomas in patients. The O2 consumption rate of the heterotransplanted tumors decreases with increasing tumor wet weight and again is comparable to that of mammary carcinomas in patients. Furthermore, it is directly proportional to the O2 availability which in turn is mainly determined by TBF. The O2 utilization of the human mammary carcinomas heterotransplanted into nude rats is already high in small tumors and increases only slightly with increasing tumor weight. From these results and taking into account an impairment of convective and diffusive O2 transport it can be concluded that the O2 consumption rate of the tumor cells is mostly determined by the the O2 supply and not by the actual demand of the cancer cells. This finding holds true for different tumor cell lines in different species.