Abstract: A mechanism for varying the lift and duration of lift of the valves (1) of an internal combustion engine comprises a primary hydraulic cam follower (4) actuated by a primary cam (5), a secondary hydraulic cam follower (8) actuated by a secondary cam (7), a housing (18) for the secondary cam follower (8) adjustable about the axis of the secondary cam (7) so that the timing operation of the secondary cam follower (8) can be varied, characterized in that the bleed of hydraulic oil from the primary hydraulic cam follower (4) is controlled through the secondary cam follower (8) and thereby the rate and timing of lift of the primary cam follower (4) to thus give a variation of the time and rate of opening and duration of opening of the valves (1).