Recent global-scale observations of the mesopause thermal structure show the altitude of the mesopause to be located regularly near 100 km and in a small minority of cases near 88 km. In this paper we use a three-dimensional nonlinear model of the middle-upper atmosphere (0--150 km) to explore the processes which lead to this two-level structure of the mesopause. More specifically, we evaluate by way of sensitivity studies the influences of (1) photochemical heating processes, (2) gravity wave momentum deposition, and (3) vibrational excitation of CO2 molecules by collisions with O atoms on the global mesopause structure. We also calculate the effects which solar-induced tides have locally on the diurnal variations of the mesopause altitude. The results of our numerical simulations suggest (1) that indeed two distinct mesopause levels exist worldwide even in terms of zonal mean states, (2) that the two-level feature is mostly due to the photochemical heating processes in combination with gravity wave momentum deposition, and (3) that the two-level mesopause structure is not a local phenomenon of tidal wave activity. ¿ 1999 American Geophysical Union