Purpose: To evaluate the effect of water deprivation on the signal intensity of the posterior pituitary gland on T1-weighted MR images and correlate the signal intensity with the plasma vasopressin concentration. Material and Methods: Fifteen rabbits were studied: Group 1 (n=10) was deprived of water for 9 days and Group 2 (n=5) was replenished water for 7 days after 7-day water deprivation. MR imaging and plasma vasopressin measurement by radioimmunoassay were made before and after water deprivation and replenishment. Sequential changes of the signal intensity ratio of the posterior lobe to the pons and plasma vasopressin concentration were correlated. Results: Before water deprivation, the hyperintense posterior lobe was demonstrated in all rabbits. During water deprivation, the signal intensity ratio decreased and vasopressin concentration increased gradually. On the contrary, the signal intensity ratio increased and vasopressin concentration decreased with water replenishment. The signal intensity ratio correlated well with the plasma vasopressin concentration (p<0.05). Conclusion: There was a negative, linear correlation between the signal intensity ratio of the posterior pituitary gland on T1-weighted MR images and plasma vasopressin concentration to water deprivation. The results support that the high signal intensity of the posterior pituitary gland on T1-weighted MR images is attributed to the normal content of vasopressin-neurosecretory granules.