Endorphin and ACTH-like materials levels in rat plasma and pituitary were measured by radioimmunoassay under baseline and cold stress conditions. Cold stress significantly increased plasma beta-endorphin and ACTH immunoreactivity. A rise in these two peptides was also found in the neurointermediate lobe of the pituitary, while in the anterior lobe their levels were unaffected. These findings suggest that the rise of beta-endorphin and ACTH content in the neurointermediate lobe occurs as a compensatory biosynthetic mechanism for the peptides released from the adenohypophysis.