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The proton NMR relaxation times of lung tissue were determined in a rabbit model of acute and evolving pulmonary hemorrhage (PH). Pure PH was simulated by injecting blood into a single lobe using endobronchial catheterization. In vitro spectroscopic measurements of T1 and T2 were made and total water content was determined on lung samples that were excised at regular intervals. T1 and T2 were markedly longer in lungs with acute PH than in normal lungs (T1: 818 ± 44 vs. 643 ± 4 msec; T2 164.0 ± 16.3 vs. 88.1 ± 3.4 msec). Within the first 24 hours, evolving PH was characterized by a rapid and progressive decrease in T1 (—50%) and T2 (—57%). Up to seven days after the instillation of blood, the T1 (450 ± 43 msec) and T2 (69.7 ±1.9 msec) of lung with modeled PH remained below values of normal lung. The observed shortening of the relaxation times of lung disease with PH was closely paralleled by a decrease in tissue water content.