In this study, we proposed a Z-spectrum fitting method based upon Lorentzian Probabilistic Sum (LPS) for computing CEST and NOE contrast. Proposed fitting method was tested on multi-pool Z-spectra data acquired using simulations and from in-vivo human brain data at 7T. Proposed fitting results were compared with asymmetry analysis and another fitting method based on Linear Sum (LS) of Lorentzian functions. Results of this study show that both LPS and LS nicely fit z-spectra; however, LPS provide more accurate estimation of NOE and CEST contrast. Therefore, proposed LPS model can be used for improved estimation of separate CEST and NOE components.