A computer simulation has been carried out to study the dynamics of pore formation in porous silicon. Porous structures were generated by a simple three-dimensional Monte Carlo calculation, based on a modified percolation model for pore growth in lightly and heavily doped silicon substrates. The results are consistent with recent observations. As an approximate guide, the type formed morphology was shown to depend mainly on the doping level of the substrate. A strong dependence of the average porosity on the doping level of the substrate is also predicted.