This paper presents an extension for linear and spherical flow conditions of the El-Khatib method for the analysis of pressure interference tests through the use of the pressure derivative. For linear flow a graph of In vertical bar root t partial derivative Delta p(wf)/partial derivative t vertical bar vs. l/t should show a straight line with slope m(L) and intercept b(L), which can be used to estimate the formation permeability and porosity; similarly, for spherical flow a graph of In vertical bar root t(3)partial derivative Delta p(wf)/partial derivative t vertical bar vs. l/t should show a straight line with slope m(sph) and intercept b(sph), which can be used to estimate the same previously discussed parameters. A friendly computer code for the automated analysis of pressure interference tests is also discussed. Two field cases are presented to show the use of this program. The first deals with the interference tests conducted in the naturally fractured Klamath Falls geothermal field, and the second is a test carried out in a river-formed bed, dominated by linear flow conditions.