Healthy volunteers will be recruited and screened for eligibility. Participants will be placed into three possible groups based on genetic information obtained during screening. Rolling admissions will continue until at least 10 participants have been recruited for each genetic group. Participants will be randomly assigned to receive either the control (propranolol and saline) or combined drug (propranolol and atropine) treatment in a non-blinded fashion. The participant will return over one week later to receive the alternate treatment. Continuous heart rate/blood pressure data will be recorded until the end of the study period. Respiratory rate will be maintained at a fixed rate. Participants will undergo an orthostasis task, receive the drug or control infusions, and blood samples will then be obtained to determine drug concentrations at specific time intervals. Several relatively new mathematical techniques will be applied to the data.