A valve train for a four-cycle, double overhead camshaft type internal combustion engine includes apparatus for movably supporting the camshafts that mount the cams which operate the intake and exhaust valves respectively so that the camshafts and cams are displaceable with respect to the rocker arms through which the operation of the cams is imparted to the valves. A control system is described that enables the camshaft supporting apparatus to be selectively displaced in response to engine operating conditions, such as rotational speed, so that valve timing and valve lift for the respective valves can be independently varied to produce optimal engine operating characteristics over a wide range of engine operating conditions. A method of operating engine valves in accordance with utilization of the described control system is also disclosed.