In the paper, a previously introduced method (Sari, 2009) is applied to the problem of estimating the rock mass properties of Ankara andesites. For this purpose, appropriate closed form (parametric) distributions are described for intact rock and discontinuity parameters of the Ankara andesites at three distinct weathering grades. Then, these distributions are included as inputs in the Rock Mass Rating (RMR) classification system prepared in a spreadsheet model. A stochastic analysis is carried out to evaluate the influence of correlations between relevant distributions on the simulated RMR values. The model is also used in Monte Carlo simulations to estimate the possible ranges of the Hoek-Brown strength parameters of the rock under investigation. The proposed approach provides a straightforward and effective assessment of the variability of the rock mass properties. Hence, a wide array of mechanical characteristics can be adequately represented in any preliminary design consideration for a given rock mass. (C) 2010 Elsevier Ltd. All rights reserved.