Guinea pig spermatozoa are able to undergo capacitation and the acrosome reaction in a K+-free (-deficient) medium. However, they are unable to fuse with eggs unless they are exposed to a millimolar concentration of extracellular K+ during or after the acrosome reaction. Apparently, the plasma membrane over the equatorial segment gains the ability to fuse with eggs in the presence of K+ during and/or after the acrosome reaction. Once it becomes fusible, the membrane retains its fusibility even in a K+-deficient medium. Rb+ is almost as effective as K+ in rendering the sperm membrane fusible. Li+ and Cs+ are less effective. The molecular mechanism by which K+ renders acrosome-reacted spermatozoa fusion competent is unknown, but it may involve K+-mediated efflux of H+ from the spermatozoa.