1. A valve operating system for the intake or exhaust valves of a multicylinder internal combustion engine, comprising, a first valve operating means for causing operation of the intake or exhaust valves in first and second modes at different engine speeds, a second valve operating means for causing operation of the intake or exhaust valves in third and fourth modes at different engine speeds, at least one of said first or second modes being different from said third and fourth modes, said first valve operating means associated with at least one cylinder, and said second valve operating means associated with at least another cylinder different from said one cylinder.