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Abstract Objective.. In the present study, we examined the behavior and state of water molecules in immature and mature rat brains by measuring the components of magnetic resonance (MR) water proton transverse relaxation time (T₂). We also performed morphological examination of immature and mature rat brains using electron microscopy (EM). We then compared the fraction of T₂ component and the EM findings. Methods.. Midbrains of male Wistar rats were examined at various time points ranging from 4 h to 12 weeks after birth. T₂ was measured by MR, and the ratios of intra- to extracellular spaces were determined by EM in each stage. Results.. T₂ consisted of two components: fast T₂ (<100 ms), and slow T₂ (>100 ms). During maturation, values of fast T₂ decreased dramatically, but slow T₂ remained constant. However, the fraction accounted for by slow T₂ decreased from 59% to 9% during maturation. Morphological examination showed that the extracellular space fraction of the midbrain decreased from 49% to 5% during maturation. Thus, morphological change correlated well with changes in slow T₂; in other words, multicomponent T₂ results showed a close correlation with tissue compartmentalization. Conclusion.. MR relaxation times obtained by means of multicomponent analysis can thus be used to measure intra- and extracellular space fractions.