The radius of investigation (r(i)) in a naturally fractured reservoir is dependent on flow time, the relative storativity of matrix and fractures and the size and shape of the matrix blocks. Not taking this into account while using the conventional radius of investigation equation developed for single porosity isotropic reservoirs can lead to significant errors. This paper presents an equation that can be used for the case of pseudo steady state (restricted) or transient (unrestricted) interporosity flow when radial flow is dominant during the test. A straightforward approach using a function Y-a permits calculating interconnected pore volume and hydrocarbons investigated during the test in the fractured and composite systems. In addition, an equation is presented for calculating the distance of investigation in those cases where linear flow (as opposed to radial) is dominant. This occurs, for example, in paleo channels of continental origin. The radius of investigation is strictly a flow equation (not a build-up equation).