Ample evidence indicates that carbonyl groups on amino acids in the selectivity filter are responsible for K+ selectivity by the KcsA potassium channel, although the mechanism of K+ selectivity is incompletely understood. Here we present results of steered molecular dynamics (SMD) simulations of ion movement through the length of the KcsA channel. Our goal was to determine whether peaks in potential mean of force (PMF) within the selectivity filter, which would indicate strong coordination of the K+, could be identified by SMD. One K+ ion was placed in position S1 of a KcsA tetramer that had been inserted in a POPC bilayer and solvated in water, and the system was minimized and equilibrated. A 5 ns simulation was initiated with a force of 1 kcal/mol/A2 for a fictitious spring and atom that drags the K+ though the channel with a speed of 10 A/ns. As the ion was moved from the central water-filled cavity in the channel through the selectivity filter, four regions of very rapid step-wise change in PMF were found in the channel. Each relative peak and subsequent rapid drop in PMF corresponds to a region in the selectivity filter that has been identified as a site for ion binding (S1, S2, S3 and S4). Another region of very rapid change in the PMF was found in a region corresponding to a ring of 4 amino acids (THR 107) where the channel is constricted to its smallest radius of about 2.5 A. This region of the protein is relatively rigid. The PMF profile of different monovalent cations and the effect of amino acid substitutions near the constricted region of the channel on the selectivity of KcsA are the subject of ongoing investigations.