Interactions between emotional stress due to fear and hypovolemic stimuli on vasopressin secretion were studied in rats. Intraperitoneally injected dextran did not significantly change plasma osmolality and arterial blood pressure but increased blood hemoglobin and plasma vasopressin level. An i.v. infused physiological solution reversed these changes. Emotional stress due to fear acquired by learning suppressed plasma vasopressin level in dextran-injected rats. Emotional stress due to fear produced by low-frequency footshocks also suppressed the increased plasma vasopressin level. These results suggest that emotional stress due to fear interacts with afferent neural signals originating from cardio-vascular volume receptors in the control of vasopressin secretion.