An electro-hydraulic lost motion system for variable valve activation including a master piston and an accumulation piston in a first bore, defining a hydraulic pressure chamber therebetween, in response to rotation of an engine cam. A slave piston in the engine head and hydraulically connected to the pressure chamber opens and closes an engine valve. A servo-valve behind the accumulation piston controls the mobility of the accumulation piston via a fluid control chamber. When the control chamber is made hydraulically rigid, the system actuates the engine valve. When the control chamber is vented through the servo-valve, the accumulation piston is movable in lost motion, preventing the engine valve from opening. All intermediate degrees of valve opening are possible. Preferably, the servo-valve, control chamber, accumulation piston, and a control piston are comprehended in a modular subassembly which may be positioned adjacent the master piston or the slave piston.