This paper studies the feasibility of improving vehicle ride comfort and vehicle dynamics by applying mechanically controlled hydropneumatic suspension to a medium-duty truck. Both front and rear suspensions consist of hydraulic cylinders, small gas accumulators and leaf springs. Hydraulic pressure In the cylinder is controlled by a hydraulic pressure difference between diagonally located hydraulic cylinders. Vehicle test results show that this suspension system reduces vertical vibration in the frequency range of 3 to 10 Hz, the pitch motion during braking, and the roll angle during a steady turn, when compared those the conventional suspension system. However, due to a response lag in the hydraulic control system, this system causes an unfavorable vehicle motion when there is a rapid steering operation, such as an abrupt lane change.