Cocaine likely creates its reinforcing and addictive effects by increasing levels of dopamine, a brain neurotransmitter. GABA agonists are chemicals that have the opposite effect by inhibiting the release of dopamine. Increasing GABA activity may result in greater inhibition of dopamine systems, which may lead to new treatments for cocaine abuse. The purpose of this study is to determine whether pretreatment with GABA agonists reduces the psychological and physiological reinforcing effects of cocaine. Specifically, the study will look at three different GABA agonists: tiagabine, baclofen, and trazolam.