Purpose: Grapefruit juice is one of the most extensively studied dietary/natural substances shown to interact with a variety of medications. However, unanswered questions remain regarding the causative ingredients and mechanisms underlying such drug-grapefruit juice interactions. Compounds in grapefruit juice called furanocoumarins have been established as major causative ingredients, which act by inhibiting the elimination (metabolism) of drugs, leading to increased circulating drug concentrations. Increased drug concentrations can in turn lead to increased drug potency or even toxicity. Grapefruit juice also has been shown, paradoxically, to decrease circulating concentrations of some drugs, including the non-sedating antihistamine agent, fexofenadine (Allegra), which undergoes negligible metabolism. Whether or not furanocoumarins mediate the decrease in fexofenadine concentrations is unknown. The purpose of the proposed study is to compare the effects of a "furanocoumarin-free" grapefruit juice with grapefruit juice on circulating concentrations of fexofenadine.