The early time response in the well testing of an homogeneous reservoir is expected to give a unit slope when log (pressure) is plotted versus log (time). It is shown that this response is a special case, and that in general another non-dimensional parameter must be defined to describe the set of curves that could take place for each value of C/sub D/. This parameter, t/sub Rw/, is the time response of the reservoir divided by the time response of the well. When t/sub Rw/ is large, a unit slope results (as in oil and gas fields where kh/..mu.. is relatively low) while no unit slope should be measured for small values of t/sub Rw/ (as in geothermal fields where kh/..mu.. is much larger than in an oil field). Using a numerical model of transient two-phase flow, the predicted early time behavior in well testing has been plotted for C/sub D/ = 25, 100, and for values of t/sub Rw/ = 10, 1, 0.1 in each case. It is shown that both C/sub D/ and t/sub Rw/ are needed to correlate the behavior. In addition, the effect of heat transfer on the downhole pressure change with time has been calculated using the assumption of thermodynamic equilibrium. If the well test data is analyzed without taking into account the heat loss, the calculations for kh/..mu.. are wrong. It is also postulated that non-equilibrium effects in the well many lead to abrupt changes in the pressure versus time curve. Such changes have been observed in field tests of flashed geothermal systems