 To compare the effects of fexofenadine HCl 180 mg, cetirizine 10 mg, and placebo on computerized scores derived from the Aeromedical Vigilance Task AVT, a computerized objective measure of attention, accuracy and reaction time in healthy naval flight personnel. Specifically, the primary objective is to compare the AVT overall average of the 18 blocks mean change from baseline in total number of errors commission errors omission errors between fexofenadine HCl and cetirizine 10 mg. Safety of these single-dose treatments also will be assessed. 