We have surveyed the sensitivity of the spin lattice relaxation rates of the 19F resonances of several perfluorocarbons to changes in oxygen tension and temperature. Hexafluorobenzene was found to exhibit exceptional sensitivity to changes in oxygen tension, and we have exploited this phenomenon to measure tumor oxygen tension following intratumoral injection. When 20 microliters hexaflourobenzene were injected they remained localized and the biodistribution was readily assessed on the basis of combined 1H and 19F three-dimensional MRI. Relaxation measurements indicated a typical baseline oxygen tension of 4.0 +/- 1.5 torr in the central region of a Dunning prostate R3327-AT1 tumor when the rat breathed 66% oxygen. Altering the inspired oxygen concentration to 100% produced a modest increase in pO2 (5.6 +/- 0.7 torr; p < 0.1). Significantly, the precision of these measurements should facilitate NMR investigations of radiobiological hypoxia. Intra-tumoral injection allowed measurements from regions not normally accessible to infused perfluorocarbons and provides an additional approach to measuring tumor oxygenation.