This is a Phase I, three period, two sequence, single-center, open-label, randomized, crossover study design. Periods I and II consist of the oral administration of either a single dose of aminophylline or ambrisentan alone, followed by a 48 hour wash out interval. Subsequently, Period III consists of the simultaneous administration of both drugs. All subjects are to be confined to the Duke Clinical Research Unit throughout all treatment periods [Study Day -1 to Study Day 6 (discharge)]. A sufficient number of healthy adult subjects will be consented in order to enroll 24 and complete 16 subjects who complete all three periods. Replacement of subjects is permitted, if necessary.