Abstract: An improved intake and exhaust valve system for internal combustion engines is disclosed. The system includes a primary valve control mechanism for sequentially opening and closing the intake and exhaust valves, and a secondary control system for modifying the operation of the primary mechanism. The primary valve control mechanism includes cylindrical cams for opening and closing the intake and exhaust valves. The secondary control system modifies the operation of the primary valve control system to vary the duration, extent and onset of valve opening and closing. The secondary control system includes a control plate, at least one rocker mechanism mounted to sequentially engage and disengage the control plate, and a driver mounted between the rocker mechanism and at least one of the cylindrical cams of the primary control system so that, when the control plate engages the rocker mechanism, the driver mechanism engages one of the cylindrical cams to modify the operation of the primary control system.