Susan,
I am lifting weights and have found it to be helpful to maintaining core strenght and physical fitness. I consulted with a PT who is PD knowledgeable. He suggested that I do one or two sets at a weight that I can handle doing 20-25 repetitions (without straining and able to complete the full range of motion for the exercise). I was diagnosed with PD a year ago and have been going to the gym ever since - I lift weights 2-3 times per week. It has really helped me physically and especially with my balance. I would consult your MD and PT before starting. I do a full body workout - working all the major muscle groups (hams, quads, glutes, abuductors, adductors, lats (back), shoulders/press, rows (back), bench (chest), curls (biceps), triceps (deltoids), and abs (stomach). I finish the gym with balance and stretching. AND make sure your DRINK WATER.
For whatever its worth,
Steve