The normal neurohypophysis is hyperintense relative to brain and adenohypophysis on T1-weighted MR images, but the signal is not chemically shifted with respect to water. The source of the hyperintense MR signal in the normal neurohypophysis has been the subject of recent controversy. To date, an adequate biophysical explanation for the unusual imaging properties of the neurohypophysis has not been found. The purpose of this study was to investigate the effect of two chemical components of the neurohypophysis, phospholipids and vasopressin, on the MR signal. We synthesized phospholipid vesicles of the same size as those found in the neurohypophysis (100-200 nm) and quantitatively measured T1, T2, and chemical shift in a spectrometer at concentrations of 0-250 mg/ml of phospholipid. Imaging of the test materials was performed on a 1.5-T whole-body MR system using T1-weighted images, T2-weighted images, reduced bandwidth, and fat suppression techniques. The experiment was also performed with saline buffer, mineral oil, vasopressin, and vasopressin incorporated into the core of the phospholipid vesicles. We found that a solution containing phospholipid vesicles has T1 and T2 characteristics analogous to the neurohypophysis and that this solution exhibits a single peak that is not chemically shifted with respect to water. Vasopressin had no effect on the signal, neither in solution nor in the vesicles. We conclude that phospholipid acts as a relaxation enhancer of water protons and that the MR characteristics of the phospholipid vesicles can account for the observed MR properties of the neurohypophysis.