Astressin is a potent corticotropin releasing factor (CRF) antagonist. In Vitro: Astressin has low affinity for the CRF binding protein and high affinity (K_i=2 nM) for the cloned pituitary receptor. Astressin shows high affinity for cloned human CRF-RA1 stably expressed in CHO cells and high potency to inhibit ACTH secretion^[1]. In Vivo: Astressin is significantly more potent than any previously tested antagonist in reducing hypophyseal corticotropin (ACTH) secretion in stressed or adrenalectomized rats. Low doses of astressin (30 μg and 100 μg per kg) administered i.v. still produce a significant decrease in ACTH levels at 45 and 90 min, respectively^[1]. Astressin significantly reverses the anxiogenic-like response induced by both social stress and ICV rat/humanCRF (r/hCRF) on the elevated plus-maze, but fails to block the effects of r/hCRF-induced locomotor activity in a familiar environment^[2]. Intracerebroventricular infusion of the peptide both 30 min before and 10 min after seizures decreases damage in some hippocampal cell fields by as much as 84%, a magnitude of protection greater than reported for other CRF antagonists against other models of necrotic neuronal injury. Astressin protects even if administered only 10 min following excitotoxin exposure^[3].