Summary This paper presents a solution for the inverse problem, or interpretation, to the flow of tracers in naturally fractured reservoirs. The model considers cubic block geometry. The Rosenbrock method for nonlinear regression used in this study, allowed the estimation of up to six parameters for this cubic block geometry. The nonlinear regression for the three cases was carefully tested against synthetical tracer concentration responses affected by random noise, with the objective of simulating as close as possible step injection field data. Results were obtained within 95 percent confidence lirnits. The sensitivity of the inverse problem solution on the main parameters that describe this flow problem was investigated. The main features of the nonlinear regression program used in this study are also discussed. The procedure of this study can be applied to interpret tracer tests in naturally fractured reservoirs, allowing the estimation of fracture and matrix parameters of practical interest (longitudinal fracture dispersivity a, matrix porosity, fracture half-width w, matrix block size d, matrix diffusion coefficient D2 and the adsorption constant kd). The methodology of this work offers a practical alternative for tracer flow tests interpretation to other techniques. Introduction Current hydrocarbons production comes importantly from naturally fractured formations. The behavior of these reservoirs is quite different from that of "homogeneous" - conventional - reservoirs. The complex matrix-fracture interaction of these systems makes their characterization a challenging task. Among the different tools currently available to accomplish this endeavor, tracer test interpretation is taking an ever increasing role. These interwell tracer tests have significantly contributed to the better understanding of the fluid flow in these systems. Radioactive and chemical tracers have been used for many years in groundwater hydrology to analyze the movement of water through porous formations, but their use in geothermal and petroleum reservoir engineering is more recent. It has been recognized, as already stated, that tracer test interpretation, in addition to well-to-well pressure transient tests, is a very important contribution towards accomplishing the characterization of naturally fractured reservoirs. As concluded by these authors, these two testing techniques are complementary, not competing. There are several papers that discuss the flow of tracers in naturally fractured reservoirs. For a review of the recent work the papers of Rarnirez et al. may be consulted. Most of these studies deal with the direct problem (i.e., predicting the tracer response behavior from the knowledge of pertinent reservoir and tracer parameters).