Xenon-133 is a widely used tracer for clearance measurements to estimate nutritive blood flow. There are however certain difficulties in handling and obtaining the radionuclide. Pertechnetate ion, 99mTcO4-, is used extensively as a radiotracer in nuclear medicine studies and is easily available. To assess the usefulness of pertechnetate ion for blood flow studies, we have measured the rate of clearance following a local injection of pertechnetate ion and xenon simultaneously in normal mouse leg muscle and in murine tumors (fibrosarcoma and mammary carcinoma) of different sizes. The partition coefficients for the two tracers were also measured. The results indicated that the clearance halftime for pertechnetate ion was consistently greater than that for xenon in the tumors but similar in normal resting muscle. The estimated blood flow values assessed using the two tracers were similar in all cases except for large tumors (greater than 1.5 g). In this case, pertechnetate ion gave lower values than xenon. The difference could be explained by the large intracapillary distances which occur in poorly vascularized regions of large tumors since pertechnetate is a charged ion and hence might be expected to have limited diffusion capacity. The results indicate that caution is warranted when pertechnetate ion is used as a tracer for measurements of blood flow and for other biological determinations which depend on tissue transport for properties.