Background: Use of Mayfield skull clamp during craniotomies; results in a sharp and intense noxious stimulus which results in a severe hemodynamic pressor response. Many strategies have been reported to blunt this undesirable pressor effect. This prospective, double blind, and randomized study is the first to evaluate the effect of MgSo4 on attenuation of hemodynamic pressor activity after head clamp application during craniotomies.