A hydrostatic power transmitting device in combination with a speed reducing apparatus together disposed within a common housing having an interior space divided by partitioning device into a first region in which the hydrostatic unit is disposed and a second region in which said speed reducer is accommodated. First and second regions are segregated from each other by the partitioning device being in the form of a flexible non-porous barrier. The elastically deformable partitioning device can therefore adjust its shape to take up any change in the amount of hydrostatic transmitting fluid in the first region due to temperature changes of the fluid and facilitates the regulation in depth of lubricant held by the second region. Thereby an initially low level of lubricant in the second region lessens the adverse effect of power-retarding drag losses, especially during cold weather winter operation, and a rising level of lubricant ensures good lubrication when temperatures are elevated and viscosity is low. Preferably the second region is sealed from the environment, and the above atmospheric positive pressure in the second chamber aided or induced by the expanding volume of fluid in the first region provides a net increase of pressure experienced by low-pressure side of the closed-loop circuit of the hydrostatic transmission.