Using i.p. delivered deuterated water, and 2H NMR spectroscopy, it is possible to non-invasively, serially, and quantitatively measure blood flow through tissue volumes as small as 0.05 ml. Measurements made with this technique show that changes in total tumor blood flow reflect local changes measured superficially by laser Doppler flowmetry, and that tumors of identical histology and size have heterogeneous blood flow rates. Finally, we have shown that tumors growing in an irradiated normal tissue have a lower blood flow rate which is manifest when tumors grow beyond 0.1 ml.