Drug addiction is a chronically relapsing disorder and heroin dependence is still a challenge with devastating impact on both the individual and the society. Medication assisted rehabilitation is offered by our health care system to treat opioid dependenc e and to prevent relapse. Opioid maintenance treatment (OMT) using methadone and buprenorphine is established and increases steadily. Antagonist treatment with naltrexone implants is a promising, but not-yet established alternative approach. Naltrexone bl ocks effectively heroin effects without inducing dependence, cognitive impairment or reward. Several studies have emphasized the role of cognitive status on drug-abuse rehabilitation outcomes because OMT may have negative effects. Morphological changes in the brain of opioid abusers have been described in a few studies, but is unclear which effect the different treatments may have and will be investigated. The availability of selective opioid tracers for use in positron emission tomography enables us to i nvestigate the effect of treatment related to the receptor occupancy of the drug in vivo in the brain. However, this is an area of scarce knowledge. Therefore, the project will use neuropsychological testbatteries combined with neuroimaging as instruments for the investigations. The same approach will be used for each of the relevant patient treatment groups (OMT with methadone, buprenorphine, naltrexone implants and opioid abstinent controls). The aim of the present project is to investigate how medicati on assisted rehabilitation affects cognitive functions and neurobiological mechanisms. The involved institutions provide an excellent enviroment to ensure a successfull implementation and performance of the project. The project will provide a better under standing of the underlying neurobiology of dependence and treatment of dependence, and, as well, have a direct impact on the choice of treatment and how each treatment may be optimized.