Use is made of the concept of nuclear average temperature'' to relate the spatial temperature profiles in fuel and graphite attained during high power operation of the MSRE to the neutron multiplication constant. Based on two-group penturbation theory, temperature weighting functions for fuel and graphite are derived, from which the nuclear average temperatures may be calculated. Similarly, importance-averaged temperature coefficients of reactivity are defined. The values of the coefficients calculated for the MSRE were --4.4 x 10/sup -5// deg F for the fuel and -7.3 x 10/sup -5/ for the graphite. These values refer to a reactor fueled with salt which does not contain Th. They were about 5% larger than the values obtained from a oneregion, homogeneous reactor model, thus reflecting the variation in the fuel volume fraction throughout the reactor and the effect of the control rod thimbles on the flux profiles. (auth)