The evolution of the three-dimensional resistive tearing instability in a magnetotail configuration is analyzed by means of a resistive MHD code for various forms of the energy equation including ohmic and compressional heating. As an introduction we present a complete derivation of the set of MHD equations from a kinetic formalism noting all simplifying assumptions. The following study emphasizes the changes in magnetotail evolution brought about by a variation of the assumed value of the polytropic exponent &ggr;, representing the ratio of specific heats in the energy equation, which closes the set of MHD equations. We find that within the range of 2/3≤&ggr;≤4, a tearing mode develops in all cases. Certain properties, such as the formation and ejection of a plasmoid, associated with the formation of the reversal region of the north-south component of the magnetic field, and strong earthward and tailward flow fields prevail as qualitative features independent of the value of &ggr; chosen. Quantitatively, however, magnetotail evolution can be quite different for different values of &ggr;. This is found primarily in the evolution of the temperature, where we find heating by a factor of order two with higher temperatures for higher values of &ggr;. This is combined with a density reduction by a similar factor. Both features are consistent with recent observational results from a superposed epoch analysis of 39 substorm events in the near tail (Baumjohann et al., 1991). Ohmic dissipation is found to be the major source of the heating, while adiabatic or nonadiabatic compression may contribute primarily in the closed field region earthward of the reconnection site. Furthermore, quantitative values of typical flow speeds and evolution time scales show some dependence on the polytropic index. These results suggest that the qualitative features of the nonlinear evolution of the resistive tearing mode are not strongly influenced by the energy equation, whereas details can show strong dependence. ¿ American Geophysical Union 1992