We have analyzed K+/Na+ selectivity of the KcsA potassium channel from the point of view of solvation structure and solvation energetics at the atomic level by using the reference interaction site model (RISM) integral equation theory. Our results indicate that K+ is better dehydrated and has lower binding free energy in the four-site selectivity filter of the KcsA potassium channel than Na+. Thus the selectivity filter favors K+ over Na+ to enter and pass through the channel. A good agreement between our energetic results and those from simulation has been obtained. (C) 2002 Elsevier Science B.V. All rights reserved.