NTP will be injected into a peripheral vein as bolus infusion starting with 50mcg and increasing in 50mcg increments. NTP administration will be stopped when a 20-30mmHg drop in systolic or diastolic blood pressure is elicited. This will be followed by PE administration as a bolus infusion starting with 50mcg and increasing in 50mcg increments. PE administration will be stopped when an increase in systolic or diastolic BP of 10-20 mmHg above baseline is elicited. If, at any time during drug infusion, systolic blood pressure decreases below 90mmHg or increases above 160mmHg, or if the subject complains of discomfort that could be associated with blood pressure changes, the infusion will stop immediately. Specific conditions will result in arterial BRG being assessed using only NTP or PE: 1) allergy to NTP, or a systolic blood pressure (SBP) <100 will exclude the use of NTP; 2) hypersensitivity to phenylephrine products, narrow angle glaucoma or a SBP >140mmHg will exclude the use of PE. Drugs will be infused through existing venous access.