A method and apparatus for conducting in situ tests on a subsurface earth formation of interest which is traversed by a wellbore. A wireline formation testing instrument is positioned at formation depth and a sampling probe thereof is extended into fluid communication with the formation and isolated from wellbore pressure. Utilizing a hydraulically energized double-acting bi-directional piston pump and by valve controlled selection of pumping direction testing fluid such as completion fluid may be pumped into the formation through the sampling probe either from fluid reservoirs of the instrument or from the wellbore. After reversing the operating and control valving of the pump and instrument, the piston pump is utilized to extract formation fluid from the formation and pump it to sample tanks of the instrument, pump it into the wellbore or subject it to controlled pressure for real time formation testing and for formation characterization. The pumping mechanism of the formation testing instrument includes a pump-through capability for unlimited pumping of formation fluid to permit effective removal of filtrate, sand, rocks and other debris until an uncontaminated formation fluid sample can be recovered for testing. The piston pump and valving arrangement further permits successive draw-down of minute volumes for rapidly determining formation pressure and permits bubble point pressure and fluid compressibility testing to enable bubble point pressure and fluid compressibility profile plots for formation characterization.