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A systematic numerical study of the macroscopic electrical conductivity of vugular porous media has been conducted by solving the Laplace equation. The structure of these bimodal media is characterized by the micro and macroporosities and by the micro and macro correlation lengths. The overall correlation function is analyzed. It is shown that mainly depends on the total porosity, and very little on the other parameters. Moreover, similar results are obtained when the microporous medium is considered as a continuum. Finally, these predictions are compared to experimental data for the formation factor and the thermal conductivity of limestones; the agreement is good in the first case, and excellent in the second.