We have modelled the effect of motor evoked potential (MEP) amplitude variation on transcranial magnetic stimulation (TMS) maps. The range of variability in TMS map parameters was estimated by randomly altering the MEP amplitude associated with each stimulus site and re-calculating the map parameters. TMS map position and area were remarkably stable, with variations of the order of 1 mm for map position and less than 5% for map area. The results indicate that reliable and accurate mapping studies can be carried out in the presence of an intrinsic variability in MEP amplitude measurements. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.