All groups will be age- and sex- matched. Pre-treatment with varenicline (VAR) or placebo will start on Day 1 of each test session will be as follows: 1) 0.0 mg/day 2) 0.5 mg twice daily 3) or 1 mg twice daily for 3 days. Testing days will be separated by at least 1 week apart to rule out medication carry-over effects.