Though oxytocin and vasopressin are similar in structure and are produced in the same brain regions, they show specific responses under stress conditions. In humans, increases in peripheral blood vasopressin appear to be a consistent finding during many acute stress situations, while in rats, vasopressin secretion is unresponsive to several stimuli known to induce ACTH and catecholamine release. Even decreases in vasopressin levels during stress were described. In accordance with others, we observed enhanced vasopressin release in response to stress stimuli with an osmotic component such as hypertonic saline injection but also during exposure of rats to a warm environment. Immobilization stress which fails to induce vasopressin release was reported to increase hypothalamic vasopressin mRNA and plasma vasopressin levels in chronically adreno-demedullated rats. Unlike vasopressin, oxytocin may be considered a typical stress hormone responding to osmotic as well as other stress stimuli. We found that acute exposure of rats to immobilization stress resulted in an increase in oxytocin mRNA level. In addition, we have shown that magnocellular neurons of the paraventricular nucleus, but not the supraoptic nucleus, are essential for oxytocin release during immobilization stress. The release of posterior pituitary hormones represents an important component of the stress response.