Blood samples for the measurement of plasma concentrations of droxidopa and metabolites including but not limited to 3-OM-DOPS, NE, vanillic acid, and protocatechuic acid will be collected before and 0, .5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, 24, 30, 36 hours after dosing for healthy volunteers and subjects with mild, moderate, and severe renal impairment and those with ESRD. For the latter, samples will be collected on both a non-hemodialysis and a hemodialysis visit.