A valve actuating system for actuating at least one valve of an engine includes an improved mechanism for varying the timing and/or lift of the valve. The mechanism uses two adjacent rockers that cooperate with two adjacent cams of a camshaft. In one operating condition, movement of a first rocker is transmitted to the valve through the second rocker and in another operating condition only the movement of the second rocker is transmitted to the valve. The rockers preferably cooperate with cams having different lifts. The lift of the cam driving the first rocker preferably is greater than the lift of the cam driving the second rocker.