Relapse is a major characteristic of drug addiction, and remains the primary problem in treating drug abuse. Without an understanding of the factors that determine renewed drug-seeking, the urge to use drugs, and the persistent craving for them, it is unlikely that health care professionals can provide effective treatment. Using an animal model of relapse, the author and her team are studying factors that induce reinstatement of drug-taking behaviour after short and long periods of abstinence, and they are exploring the neurobiological basis of these effects. In their experiments, rats are trained to self-administer drugs intravenously by pressing 1 of 2 levers. During a subsequent period, the drug is no longer available, but the rats are free to try to obtain the drug (a period of "extinction training"). After extinction of responding, the investigators test for the ability of various events to reinitiate drug-seeking. On this background of renewed drug-seeking or relapse, the investigators search for pharmacological and neurochemical manipulations that might block or attenuate such behaviour. They have found that the 2 most effective events for reinstating responding after both short and long drug-free periods are re-exposure to the drug itself and exposure to a brief period of stress. The critical neurochemical pathways mediating drug-induced relapse are not identical to those mediating stress-induced relapse. Relapse induced by "priming" injections of heroin or cocaine involves activation of the mesolimbic dopaminergic pathways, whereas relapse induced by stress involves actions of corticotropin-releasing factor (CRF) in the brain, and of brain noradrenergic (NE) systems. In addition, evidence shows that CRF and NE may interact at the level of the bed nucleus of the stria terminalis in stress-induced relapse. By contrast, relapse induced by "priming" injections of drugs is relatively unaffected by manipulation of CRF and NE systems of the brain.