Our recent simulations of the selectivity of the filter of a calcium channel are reported and discussed briefly. The model calcium filter that is used is a smooth cylinder embedded in a membrane with smooth surfaces. The four glutamate side chains of the calcium channel filter are modeled as eight half-charged oxygen ions that are confined to remain within the filter but are otherwise mobile. This model filter has a strong preference for calcium ions over sodium ions because of the ability of a calcium ion to deliver twice the charge of a sodium ion while occupying the about same volume and so satisfy local charge neutrality in the confined space of the filter more efficiently. (C) 2002 Elsevier Science B.V. All rights reserved.