Nuclear magnetic resonance transverse relaxation time (T2) was previously measured in studies of lung water. The T2 decay curves for peripheral lung tissue were found to be multiexponential with two T2 components: T2 fast (T2f) and T2 slow (T2s). This behavior was explained by the compartmentalization of water, in which the protons of water are restricted and do not undergo rapid exchange between the compartments. We investigated the origin of the water for these T2 components using excised rat lungs. The effect of magnetic field inhomogeneity due to air-tissue interfaces was examined by degassing some lungs. The contribution of intravascular water was examined by perfusing the lungs with oil or NaCl solutions. Degassing produced a greater increase in the T2f than the T2s component, indicating that the water in the alveolar walls exposed to air spaces contributed to the T2f. Perfusion with oil decreased the T2s, indicating that intravascular water contributed to the T2s component. The effects of intravascular osmotic pressure on the T2f and T2s components suggest that intracellular water is related to the T2f component.