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lj,   Thanks for your question.  The prone position is the patient lying flat on the chest and abdomen (face down).  The supine position is lying flat on his/her back (face up).  There is no effect on blood pressure in either position, and there is no change in blood pressure when changing from prone to supine.   There may me a small change in pressure when assuming the sitting position, and then again when standing.  If the blood pressure drops at least 15 points (millimeters), this is considered abnormal.  If the pressure drops significantly, it could cause symptoms of dizziness or light-headedness, and if severe, patients may even pass out when they stand up.  This could be caused by medication or an underlying medical condition.  If a patient is experiencing any of these symptoms, he/she should see their physician for a thorough evaluation, as well as blood pressure readings in different positions.    I hope this has been helpful.   Martin Cane, M.D.