Vasopressin peptide will be analyzed by radioimmunoassay of vascular tissue extracts. The tissue will be extracted by first rinsing the tissue free of blood with 0.1 N Acetic Acid, blotting and weighing the tissue. The specimen will placed in cold 1.0 N HCl (1ml/0.5 g tissue) and homogenized with a Polytron for 1 min. The homogenate will then be centrifuged at 27,000 G for 45 min. The supernatant will be saved, and the pellet resuspended and rehomogenized in 1 ml 1.0 N HCl, and centrifuged. The supernatants are then combined. The supernatants will then be extracted as routinely done with plasma samples by absorption on to octadecylsilane cartridges (SepPak, C-18, Waters, Milford, MA) and eluted with acidified ethanol. The eluate is dried by vacuum and suspended in assay buffer. At this point the sample can be applied to C-18 HPLC column and eluted in a single phase buffer system consisting of 0.05 M NH4AC in 39% MeOH, and each 1-ml fraction, dried under vacuum, and suspended in radioimmunoassay buffer and assayed. Alternatively, the HPLC fractionation may not have to be done after initial confirmation that no interference is present in the initial sample. In this case, the initial extract will be assayed by radioimmunoassay directly.