1. Anterolateral cut (ALC) of the medial basal hypothalamus (MBH) in rats produces an elevation of plasma catecholamine levels, especially of norepinephrine (NE), in unstressed animals and a more pronounced rise of plasma NE levels in response to immobilization (IMO). Animals with ALC have a destroyed corresponding vasopressin (AVP) and other peptides containing innervation of the median eminence and the posterior pituitary, resulting in the prevention of increased AVP secretion during the early intervals of IMO. 2. The administration of AVP (Pitressin, 7 days, 1 IU per rat i.m.) or of 1-deamino-8-D-arginine-vasopressin (DDAVP), an AVP analogue without pressoric activity, taken in drinking water (about 100 micrograms per day) was almost equally potent in decreasing the elevated water consumption and plasma NE levels in unstressed rats with ALC. However, the stress-induced potentiation of plasma NE levels in rats with ALC was not influenced by AVP substitution and only partly reduced by DDAVP in the late IMO intervals. 3. The lack of circulating vasopressin is the main factor in the mechanism of increased activity of the sympathoadrenal system induced by ALC in unstressed rats. 4. The regulation of sympathoadrenal activity by vasopressin and DDAVP in rats with ALC seems to be mediated predominantly by V2-subtype receptors. 5. In stressed rats with ALC the potentiation of plasma NE levels was not reduced after AVP or DDAVP administration, suggesting that some addition regulatory mechanisms were involved.