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Build up test analysis for the horizontal well is based upon the theory of fluid flow in the reservoir, which is mathematically described by the diffusivity equation of the three dimensional flow and its solutions. According to the horizontal well model in the infinite reservoir for the case of the unsteady flow, four regimes can be developed: early radial flow, early linear, late pseudo-radial and late linear flow. Solutions of the diffusivity equation, that are used in the build up test analysis, are developed for each of the flow regime, supposing that the well is totally horizontal and placed in the totally homogeneous reservoir with equal formation thickness. In addition, it is considered that the reservoir is isotropic in the horizontal plane, but with the vertical anisotropy, meaning that the vertical permeability differs from the horizontal. Example of the build up test analysis in the computer program Saphir is shown for the horizontal well at the gas field Okoli. Well test data are used in the analysis as the basis for the corresponding well model design. Test data application in the well modelling is significant due to the accuracy that those models can obtain in the determination of the key parameters of the production system forecasting, such as permeability, skin factor, initial reservoir pressure etc. (the paper is published in Croatian).