The spin-lattice relaxation time $T_1$ of ${\mathrm{Xe}}^{129}$ has been measured as a function of temperature in the liquid and as a function of pressure in the gas. A strong shift $\Delta H$ in the external field required for resonance at constant frequency but varying sample density $\rho$ has been discovered. As the sample density increases in the region above 48 atm, $\Delta H$ decreases linearly at the rate of 3.45 milligauss per amagat (density at standard conditions) in a field of 8060 gauss. In the liquid the temperature dependence of the product of $T_1$ and $\rho$ can be described by an activation energy of 0.7±0.1 kcal/mole. In the gas at room temperature between 48 and 73 atm, $T_1$ varies as ${\rho }^{-2.1\mathrm{}\pm 0.4}$. The largest value of $T_1$ observed was 2600±600 sec for a gas sample at 48 atm, and the shortest value was 57±2 sec in the liquid at -101°C. The experimental values of $T_1$ have been compared with theoretical predictions in two limiting cases, the rare gas and the dense liquid. In both cases the experimental values, although larger than previously reported values, are still two to three orders of magnitude smaller than predicted from an intermolecular nuclear magnetic dipole interaction. The relaxation time was found to be independent of field. Implications of our data for determining the relaxation mechanism are discussed. It is suggested that the relaxation may be caused by a fluctuating magnetic field at the nucleus resulting from the motion of nonsymmetrical electronic charge distributions during collisions.

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