Since the discovery of opiate receptors in the central nervos system (CNS)1-3, it has become apparent that edogenous opiate ligands are involved in CNS function. Most attention has focused on their role in modulating pain, but they have also been implicated in various physiological functions and in disease states. We are concerned with evidence that endogenous opioid peptides may also contribute to the neurological deficits arising from cerebral ischaemia4,5. Dynorphin, which is widely distributed in the brain and pituitary6,7, has been reported to produce unusual motor and behavioural effects8-11 and may act as a regulatory neuropeptide, not as a classical opiate agonist or antagonist12-15. We have therefore administered to cats in which the right middle cerebral artery had been occluded both dynorphin (1-13) and analogue and control materials. We find that dynorphin (1-13) prolongs survival.