This paper presents estimates of the undisturbed formation temperatures in a geothermal exploration well drilled in the Ceboruco area in the western part of the Mexican Volcanic Belt. The method used assumes that during the warming-up period following fluid circulation, heat transfer from the surrounding formation to the wellbore is spherical-radial in the bottornhole region of the well. According to this model, the transient temperatures display a linear T versus 1/root t behavior, with an intercept equal to the undisturbed formation temperature. (c) 2005 Published by Elsevier Ltd on behalf of CNR.