A mechanism for actuating a poppet valve (28) of an internal combustion engine, comprises two coaxially mounted cams (12,14), cam followers (18,20) in contact with the respective cams (12,14) mounted on a common summation lever (16) which is movable in proportion to the instantaneous sum of the lifts of the two cams (12,14), and a rocker (24) pivotably connected to the summation lever (16) and serving to open the engine valve (28). The rocker is pivotable about a pivot (126) having a fixed preset position and a control spring (134) is provided between the summation lever (16) and a fixed point on the engine to urge the rocker (24) into contact with the tip of the valve (28) and the pivot (126) while the valve is closed.