In chickens, hyperosmolality stimulates the secretion of vasotocin (AVT) and up-regulates hypothalamic AVT gene expression. Hemorrhage, on the other hand, has not been considered an effective stimulus for AVT release in this species. The effects of acute osmotic stress and prolonged hemorrhagic hypotension on AVT gene expression and secretion were studied in White Leghorn hens. Conscious hens were osmotically stimulated by administering a single ip injection of 3 M NaCl (5 ml/kg). Urethane-anesthetized hens were bled to a mean arterial pressure of 80-90 mm Hg and the pressure was maintained within this range by additional bleeding. A total of about 30% of the estimated blood volume was removed. Both experiments were terminated after 1 hr of stimulation. Plasma AVT levels in the hyperosmotic and hypovolemic hens were 4- and 2-fold higher, respectively, compared to controls. Hypothalamic AVT mRNA levels, detected by Northern blot analysis, were 2.5- and 2-fold higher in the osmotically stimulated and hypotensive groups, respectively, compared to control groups. As determined by in situ hybridization, both osmotic stimulation and hypovolemia resulted in an increase in the number of AVT mRNA-containing neurons in the supra-optic and paraventricular nuclei. Our results indicate that, under the conditions used, hypotension and hyperosmolality are equally effective in stimulating AVT gene expression and secretion of AVT.