In previous experiments by the authors, in which hyperpolarized 129 Xe was dissolved in fresh blood samples, the T 1 was found to be strongly dependent on the oxygenation level, the values increasing with oxygenation: T 1 was about 4 s in deoxygenated samples and about 13 s in oxygenated samples. C. H. Tseng et al. (1997, J. Magn. Reson. 126, 79 -86), on the other hand, recently reported extremely long T 1 values using hyperpolarized 129 Xe to create a "blood foam" and found that oxygenation decreased T 1 . In their experiments, the continual and rapid exchange of hyperpolarized 129 Xe between the gas phase (within blood-foam bubbles) and the dissolved phase (in the skin of the bubbles) necessitated a complicated analysis to extract the effective blood T 1 . In the present study, the complications of hyperpolarized 129 Xe exchange dynamics have been avoided by using thermally polarized 129 Xe dissolved in whole blood and in suspensions of lysed red blood cells (RBC). During T 1 measurements in whole blood, the samples were gently and continuously agitated, for the entire course of the experiment, to avert sedimentation. Oxygenation was found to markedly increase the T 1 of 129 Xe in blood, as originally measured, and it shifts the RBC resonance to a higher frequency. Carbon monoxide has a similar but somewhat stronger effect.