The purpose of this study was to determine the effects of phenytoin on smoked cocaine-base self-administration using our laboratory self-administration model. A total of 12 patients randomized, 6 to phenytoin and 6 to placebo treatment group. There were 2 phases in this 10-day inpatient study. Phase 1 subjects underwent one cocaine self-admin session. Randomization took place at the start of Phase 2, Day 4 where subjects received either placebo or phenytoin. The subjects assigned to phenytoin treatment received an oral loading dose (20mg/kg) aimed at achieving plasma phenytoin concentration of (10-20 mg/L). During Phase 2, subjects had the opportunity to self-administer cocaine on Days 5, 7, and 9. The plasma phenytoin levels were expected to decrease gradually during Phase 2 which provided an opportunity to study the effect of decreasing phenytoin plasma concentrations on cocaine self-administration.