Magnetic resonance imaging of the spine was performed in six healthy male volunteers before and after 5 weeks of continuous bed rest. Imaging studies consisted of a single 1-cm sagittal section obtained with a spin-echo technique through the center of the spinal column. The T2s of the lumbar vertebral body and nucleus pulposus and the area of the latter were measured. In both vertebrae and disks, there was a significant decrease in T2 after bed rest. The nucleus pulposus also decreased in size with bed rest. The decrease in relaxation time of the lumbar vertebrae could be explained by the replacement of hematopoietic marrow by fatty marrow, a known consequence of paralytic immobilization. The decreases in size and T2 of the disks probably represent loss of water. The significance of these changes to the mechanical integrity of these structures after immobilization or space flight is not known but will depend in part on whether changes are progressive with increasing length of immobilization and on the rate and extent that they are reversed after reambulation. These results indicate that relaxation times can be altered by simple disuse, which often accompanies the underlying disease.