Magnetic resonance imaging (MRI) was used to measure the changes in the volume (and fluid content) of the lumbar intervertebral discs (L1-L2, L2-L3, L3-L4, L4-L5) in five normal subjects. For each subject, MRI scans were taken at the end of a normal day and again on the following morning (after a night's bed rest). Ten further scans were taken during an 8-h protocol consisting of alternate periods of walking (40 min) and scanning (10 min). On average, 1) disc volume increased by 10.6% during overnight bed rest, which corresponds to a gain of about 0.9 cm(3) of fluid; 2) the rate of disc volume decrease during the 8-h walking protocol was 0.96 x 10(-3) cm(3)/min; and 3) after 8 h (using our walking/scanning protocol), the disc volume did not decrease to the volume measured at the end of the previous day.