Serum dopamine—beta—hydroxylase (DBH) activity, which appears to reflect peripheral sympathetic nerve function, was measured in rats subjected to immobilization for two hours daily over a period of four weeks. Both serum DBH and blood pressure levels increased significantly after 2 weeks of repeated immobilization. The difference in blood pressure levels between control and experimental animals remained the same under anesthesia. After 4 weeks of immobilization, there was a further increase in both serum DBH and blood pressure levels. Upon cessation of immobilization intervals, serum DBH returned to normal within five days but blood pressure remained elevated for 2 additional weeks before returning to normal. (Endocrinology92: 953, 1973)