Droxidopa has been approved by the FDA for the treatment of low blood pressure in Parkinson's disease. However, as norepinephrine is also important for a lot of processes that happen in the brain as well, we believe that this medication may be also helpful for some of the other symptoms of PD. In particular, norepinephrine plays a key role in brain networks that are important for attention, decision making, and controlling movements and actions. In order for norepinephrine to reach the brain, it must cross the blood-brain barrier. Therefore, in this study we will be giving droxidopa along with carbidopa, which stops your body from breaking down norepinephrine in the blood stream and allows it to get into the brain. This is a medication that is often given in Parkinson's disease along with levodopa in the form of carbidopa-levodopa, or Sinemet. This medication works the same way with levodopa in helping it get into the brain and improve the symptoms of PD. The only difference is that levodopa works like the chemical dopamine, whereas droxidopa works like norepinephrine. Up to this point, we have not had a way to correct the low norepinephrine levels in Parkinson's disease. Therefore, this study gives us the chance to investigate the effectiveness of a potential new treatment for PD patients.