The considerable theoretical work done on the thermal behavior of the ionospheric plasma by Hanson, Dalgarno, Geisler and Bowhill, and other authors is extended here from the steady-state solutions found by them to a time-dependent solution that allows the examination of the temperature changes in the ionosphere at dawn. Physical consideration of this problem leads to models that can be mathematically represented by a second-order, nonlinear, partial differential equation, the numerical integration of which presents serious stability difficulties. An unconditionally stable form of the equivalent difference equation is used in solving numerical examples worked out for models that attempt to represent the dawn ionosphere at summer and winter for both solar cycle maximum and minimum conditions, and the following conclusions are drawn: (a) very substantial heating occurs in the ionosphere before any perceptible increase in ionization. Experimental evidence shows that a measurable increase of ionization begins at a solar zenithal angle of about 95°, while considerable heating starts at angles larger than 110°. (b) As the sun begins to shine on the high atmosphere, the competing effects of increasing heat production and growing heat capacity of the electron gas result in an electron gas temperature that initially rises fast, reaches a peak, and then declines. This peak is more pronounced the higher the altitude and predominates in the winter when the electron concentration buildup is faster. (c) Owing to smaller electron concentration in the summer, the temperatures during this season are higher than in the winter. (d) The much slower rate of increase of electron concentration during the solar cycle minimum period results in a tendency for the temperatures in this period to be higher than during the solar cycle maximum. (e) In most models it is observed that up to the time when the temperature reaches its peak, it is practically height independent at levels above the altitude of maximum heat production. Later a ‘bulge’ is formed at the altitude of maximum heat production, and a negative gradient of temperature exists in the higher levels resulting in an upward heat flow.