The origin of the hyperintense signal (HIS) of the pituitary gland posterior lobe (PL) on T1-weighted magnetic resonance (MR) images was investigated. Six rabbits were imaged on a 1.5 T device before and after 2 weeks' feeding of hypertonic saline solution (phase I). Two were killed soon after the second imaging; the pituitary gland was examined histochemically for neurosecretory granules containing antidiuretic hormone (ADH). The other four rabbits were given regular water for another 2 weeks (phase II) and then were again imaged. The plasma ADH level was monitored at each imaging session. In all subjects, the signal intensity of the PL decreased at the end of phase I, and the HIS reappeared at the end of phase II. The neurosecretory granules in the PL significantly diminished in number in phase I. Plasma ADH levels significantly increased in phase I and returned to their normal range in phase II. These clear correlations between MR findings and hormonal and histochemical results strongly suggest that the source of the HIS of the PL is the neurosecretory granules containing ADH.