To elucidate the effect of acute behavioral stress on plasma and cerebrospinal fluid (CSF) concentrations of adrenocorticotropic hormone (ACTH) and β-endorphin, rhesus monkeys were subjected to 30 min of confinement stress. Simultaneous plasma and. CSF samples revealed no significant change in CSF ACTH or β-endorphin up to 120 min after the onset of the stress despite significant elevations in plasma cortisol, ACTH, and β-endorphin. It is suggested that acute behavioral stress does not alter CSF ACTH or β-endorphin, and that this information may be clinically useful for future human studies of CSF ACTH and β-endorphin in neuropsychiatric illne