  We study the effect of a single excluded site on the diffusion of a particle
undergoing random walk in a d-dimensional lattice. The determination of the
characteristic function allows to find explicitly the asymptotical behaviour of
physical quantities such as the particle average position (drift) and the mean
square deviation. Contrarily to the one-dimensional case, where the average
coordinate diverges at infinite times as t**1/2 and where the diffusion
constant D is changed due to the impurity, the effects of the latter are shown
to be much less important in higher dimensions: for d=842, the position is
simply shifted by a constant and the diffusion constant remains unaltered
although dynamical corrections (logarithmic for d=2) still occur. Finally, the
continuum space version of the model is analyzed; it is shown that d=1 is the
lower dimensionality above which all the effects of the forbidden site are
irrelevant.
