<qa>  What is the purpose of this multi-center, randomized, double-blind trial?The purpose of this multi-center, randomized, double-blind trial is to demonstrate that paramedic initiation of the neuroprotective agent magnesium sulfate in the field is an effective and safe treatment for acute stroke. This study will analyze magnesium sulfate, an experimental therapy for stroke, versus placebo among ambulance-transported patients with acute stroke. This trial will also demonstrate that paramedics can safely, effectively, and rapidly start neuroprotective therapies for stroke. 