ABSTRACT A general method of analysis for pressure transient tests is presented. This technique is based on the pressure response of an instantaneous source and it provides a mean to compute the first and second derivatives of the influence function (unit flow rate response) of the well-reservoir system. This information is basic in identifying the flow regimes occurring during the test. This method eliminates the effect of producing time on pressure buildup data. An explicit and stable procedure is discussed to compute both the derivatives of the influence function and the initial reservoir pressure. This technique is suitable for DST and Repeat Formation Tests as well as for pressure buildup and fall off tests with long shut in period; variable flow rate before shut in can be taken into account. Examples of application are presented.