This paper presents an investigation of the torque losses in a hydraulic motor of radial piston type commonly used in industrial hydrostatic transmissions. When the speed of the hydraulic motor is decreased an increase of the torque loss can be measured. In order to investigate the reason for this increase a combination of experimental and theoretical studies is performed with special focus on two sliding contacts-the distributor valve and the piston-cam roller contact. The theoretical analysis of the contacts reveals that the distributor valve contribute more to torque and power losses compared to the piston-cam roller contacts and largely to the external leakage of the motor. At low speeds the piston-cam roller contact will enter the mixed lubrication regime.