To investigate possible sources of the low-latitude midnight temperature maximum (MTM), a series of simulations was made with the National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). The MTM is a tidal phenomenon and thus is ultimately due to the thermospheric tidal sources: in situ thermal excitation; ion-neutralmomentum coupling; and the tidal waves arising in the lower atmosphere which penetrate the thermosphere. In the TIEGCM, the development of the MTM is due to the upward propagating semidiurnal tides. The model simulations suggest that the observed seasonal variation of the MTM, in which the MTM occurs earlier and more strongly in the local summer hemisphere, may be due to the interaction of the 2,2 and 2,3 modes: in summer, the two modes reinforce each other, and in winter they offset each other. This study further suggests that much of the variability observed in the MTM may be due to the variability in the two upward propagating tides. The semidiurnal modal mix impinging on the lower thermosphere may vary on a day to day basis, depending on the background mean temperature and winds. ¿ American Geophysical Union 1996