AbstractEvaluation of water diffusion in the brain has revealed both fast‐ and slow‐diffusing water populations. It has been suggested that these populations represent extra‐ and intracellular water, respectively. We have identified and characterized both populations in the intracellular space of the Xenopus oocyte. We have also determined their T1 and T2 relaxation properties. The fast and slow intracellular populations have diffusion coefficients of 1.06 ± 0.05 μm2/ms and 0.16 ± 0.02 μm2/ms, respectively, with the fast fraction representing 89% ± 1% of the total water signal. These values are quite similar to those for total water in brain and are observed in the absence of signal from the perfusate (extracellular) water population. Volumetric swelling (16% ± 4%) of the oocyte in hypoosmotic media increased the diffusion coefficients of both intracellular populations (fast = 1.27 ± 0.03 μm2/ms, slow = 0.22 ± 0.02 μm2/ms), but did not change their relative signal fractions. This phenomenon runs counter to the effects observed in brain injury, following which the apparent diffusion coefficient (ADC) decreases 30–50%. The results presented herein suggest that this ADC decrease in brain occurs despite cell swelling, which by itself would be expected to induce an increase in intracellular diffusion coefficients. Magn Reson Med 48:765–770, 2002. © 2002 Wiley‐Liss, Inc.