From the balance between the generated heat in a gear box and the dissipated heat from the gear case surface the mean value for the expected lubricant temperature can be evaluated. The maximum oil temperature in a splash lubricated enclosed gear drive hits the transmittable power. High oil temperatures idluence wear@ scuffing@ micropitting and pitting load capacity of the gears as weil as the gear oils' service life. Experimental investigations of no-load and load dependent gear losses in cylindrical and bevel gears as a function of lubricant type and viscosity@ load@ speed and temperature are reported. The mean value for the coefficient of friction in a gear mesh is evaluated and compared to measurements in twin disk machines. A rating method for gear mesh power loss is derived. Investigations@ using model and actual gear boxes@ show the influence of radiation@ free and forced convection as weil as conduction to the foundation on the heat dissipation from the gear case surface. The influence of design@ size@ airvelocity@ surface finish of the housing@ etc.@ is reported. The results of such investigations together with accepted rating methods for bearing and seal power loss are introduced into a calculation method for the evaluation of the oil temperature in the sump of a gear box. in sample calculations the possibilities as well as the limits of this thexmal rating method is shown.