Armed services personnel in the 1991 Gulf War were exposed to a complex mixture of chemicals, including organophosphate insecticides, smoke from oil well fires, fuel oil, sarin gas, DEET, pyrethrums, and pyridostigmine bromide. Approximately 15% became chronically ill with a variety of symptoms that did not add up to a known disease. Nervous system symptoms such as difficulties with memory, thinking, and mood are commonly reported. Conventional medical practice has little to offer the ill veterans of the Gulf War. Pharmaceuticals have demonstrated neuroprotective effects in animal models and might provide symptomatic relief to ill Gulf War veterans, and these include naltrexone and dextromethorphan. Naltrexone is a chemical in this class that is a readily available generic drug with few side effects. Naltrexone is used to reverse effects of heroin and other opioid drugs. This study will determine in a scientific fashion if naltrexone and dextromethorphan are effective in treating ill Gulf War veterans. The study will be double-blinded, in that neither the veterans nor the physician treating them will know if the veterans are getting naltrexone, dextromethorphan, or a placebo pill with no medical effects. Each veteran who participates in the study will take naltrexone for three months, dextromethorphan for three months, and a placebo for three months. There will be a one-month period with no medication between each course of treatment. At the beginning and end of each course of therapy, veterans will be assessed with a medical history and physical examination. Symptom scores will be monitored using standard medical questionnaires. The veterans will take a simple computer test to determine if their brains are functioning well. They will be assessed by the investigators for clinical improvement. These will be given at the beginning and end of each course of therapy. Blood samples will be drawn to determine if certain chemicals are elevated that rise in the blood near inflammation in the body. To be eligible to participate in the study, veterans will have been in the Gulf during the Gulf War. They must have become ill immediately following this service, with the types of symptoms associated with service in the Gulf. They must continue to be ill to this time. After this study is finished, we will know whether naltrexone or dextromethorphan are effective drugs for treating ill Gulf War veterans.