The trial is a multicenter randomized, double-blind, parallel group, dose-ranging, effect-controlled study examining the effects of sodium nitroprusside in pediatric subjects requiring relative hypotension during a surgical or medical procedure. The goal is to establish the starting and maximum infusion rates that afford optimum blood pressure control in children and a safe dosing regimen in children. The objective is to describe the relationship between the infusion rate of nitroprusside and changes in blood pressure.