We present the first study of the local time and altitude variation of the MDM, the density component of the equatorial midnight pressure bulge, an important feature in the nighttime motions of the ionosphere-thermosphere system in the equatorial region. The neutral density data of the San Marco 3 (SM3) and San Marco 5 (SM5) satellites were averaged to obtain 24 hr density variations from April to December in 1971 and 1988. From these variations, the MDM amplitude as a function of altitude and local time was obtained at altitudes from 220 to 400 km at the geographic equator. We show the first evidence of downward phase propagation of the MDM together with a vertical structure not observed before, which may suggest an ionospheric interaction or perhaps a viscous dissipation effect. In 1971, the MDM propagates rather gradually from about 350 to 220 km in about two hours. In 1988, there appear two regions in which the MDM time shows little change with height with a discontinuous jump in between the two regions. The net effect is still a downward displacement of the MDM with local time. The results show significant seasonal and solar activity effects. During low solar activity periods (1971), the MDM occurs earlier in equinox than in solstice for all altitudes, consistent with the seasonal variation of the midnight temperature maximum (MTM). However, with higher solar activity (1988) and at altitudes below 340 km, the MDM occurs earlier in solstice than in equinox, raising new questions on solar activity effects on the relative phases of the tidal oscillations of the neutral density and the neutral temperature.¿ 1997 American Geophysical Union