In this paper initial results obtained from computer modelling of Motor Cortex Stimulation for chronic pain treatment are presented. The model predicts that the thickness of the cerebrospinal fluid under the epidural electrode has a great influence on the electrical field in the motor cortex and therefore on fibre stimulation thresholds. It is further concluded that afferent fibres are easier to excite than efferent fibres of the same diameter, implying that their stimulation may be responsible for the analgesic effects. However, additional anatomical data and improvements of the volume conductor and fibre model are needed to allow a better prediction which neural structure responds best to a specific stimulation. Ultimately, this modelling study should help to understand the mechanisms of motor cortex stimulation and help to improve its efficacy.