An analysis of the kinetic energy budget is made for two intensely developing cyclones over North America. The principal kinetic energy source for the first cyclone is the net horizontal transport of kinetic energy across the boundaries of the region enclosing the cyclone. For the second cyclone, it is the local kinetic energy generation. By investigating the kinetic energy budget of the vertically averaged flow (barotropic part) and the vertical shear flow (baroclinic part) it is found that the horizontal transport contribution to the kinetic energy budget of the first cyclone is evenly divided between the barotropic and baroclinic components. However, the kinetic energy generation is the dominant energy source of the second cyclone, and the horizontal transport is an energy sink. The vertical shear kinetic energy reservoir did not act as a “catalyst” as in hemispheric studies but varied during cyclone development.