Measurements of electron temperature and ion density made by langmuir probes on Atmosphere Explorer Cbetween December 1973 and December 1974 have been used to investigate the altitude dependence of the relationship between these parameters in the sunlit thermosphere. The data from 1600 perigee excursions provided measurements between 130 and 400 kilometers altitude, between 50 North and South latitude, and for solar zenith angles of less than 85¿. Within these bounds we find that the temperature and density exhibit a well defined relationship that varities with altitude. At altitudes below 200 kilometers where electrons are cooled primarily by collisions with the neutral gas, the electron temperature is nearly independent of the plasma density, both parameters increasing with altitude. At higher altitudes where electrons are increasingly cooled by collisions with ions, the temerature and density begin to exhibit an inverse relationship. The uniqueness of this behavior over a wide range of latitudes, longitudes, local times and seasons leads us to suggest that this model may be useful in estimating the electron temperature whenever measurements of plasma density are available from the other sources. We also plan to use the model to identify within the AE-C data the existence of geophysical factors that affect the electron thermal balance. However, the uniqueness of the T3(Nj, H) relationship implies that spatial and temporal variations of the heat sources and sinks may not be large for daytime nonauroral conditions.