This double-blind, 12-week trial will include heroin-dependent patients who completed detoxification. Participants will be randomly assigned to one of three conditions: naltrexone and placebo, naltrexone and memantine (15 mg bid), or naltrexone and memantine (30 mg bid). Naltrexone will be taken 3 times each week at the clinic, while memantine or placebo will be taken at home. In addition, twice each week patients will receive a psychosocial intervention that will include motivational interviewing and cognitive-behavioral relapse prevention. The goal of the psychosocial intervention is to improve compliance with medication and maintain abstinence. Baseline assessments will be taken and compared to those completed at study visits, which will occur 3 times each week.