Abstract: A downhole drill bit drive apparatus comprises a tubular housing adapted to be secured to the downhole end of a drill string, a mandrel concentrically disposed within the housing and having one end adapted to be secured to a downhole rotary drive and an other end adapted to be secured to a drill bit, the housing and the mandrel defining a fluid flow passage for transmitting drilling fluid to a drill bit secured to the mandrel, bearing assembly disposed between the housing and the mandrel for permitting rotation of the mandrel within the housing and transmitting axial and radial loads between the housing and the mandrel, a first seal at one end of the bearing assembly defining one end of a bearing chamber for housing the bearing assembly and a lubricating fluid for lubricating the bearing assembly and being operable to seal the chamber from the fluid flow passage and maintain the pressure in the flow passage and in the chamber substantially at equilibrium, and a second seal at the other end of the bearing asse