Ciprofloxacin is moderate inhibitor of cytochrome P450 1A2 (CYP1A2), one of the enzymes responsible for the metabolism of quinine. This study will evaluate the effect of ciprofloxacin-related inhibition of CYP1A2 on the pharmacokinetics of quinine sulfate. In the morning on study Day 1 after a fast of at least 10 hours, twenty-four healthy, non-smoking, non-obese, non-pregnant adult volunteers between the ages of 18 and 45 will be given one oral dose of quinine sulfate (2 x 324 mg capsules). Fasting will continue for 4 hours after the dose. Blood samples will be drawn from all participants before dosing and for 36 hours post-dose at times sufficient to adequately define the pharmacokinetics of quinine sulfate. A 7-day washout period will be completed after the first dose of quinine sulfate on Day 1. Beginning at 07:45 am on Day 8 and continuing through Day 10, all subjects will return to the clinic for non-confined dosing of ciprofloxacin (1 x 500 mg tablet) every 12 hours. Administered ciprofloxacin doses on these days will not be in a fasted state. At 07:45 am on Day 11 after a fast of at least 10 hours, all study participants will receive a co-administered single oral dose of quinine sulfate (2 x 324 mg capsules) and ciprofloxacin (1 x 500 mg tablet). A final dose of ciprofloxacin (1 x 500 mg tablet) will be administered 12 hours later. Blood samples will be drawn from all participants before dosing and for 36 hours post-dose at times sufficient to adequately define the pharmacokinetics of quinine sulfate. Fasting will continue for 4 hours following the co-administered dose of quinine sulfate and ciprofloxacin. A further goal of this study is to evaluate the safety and tolerability of this regimen in healthy volunteers. Subjects will be monitored throughout participation in the study for adverse reactions to the study drug and/or procedures. Seated blood pressure and pulse will be measured pre-dose and at 1, 2 and 3 hours post-dose on Days 1 and 11. An electrocardiogram (ECG) will be done pre-dose and at 1, 2 and 4 hours post-dose on Days 1 and 11. All adverse events whether elicited by query, spontaneously reported, or observed by clinic staff will be evaluated by the Investigator and reported in the subject's case report form.