Abstract— (1) Nerve‐ending particles isolated from crude mitochondrial preparations from rat brain by discontinuous Ficoll density gradient ultracentrifugation were shown to possess a Mg2+ and energy‐dependent transport system for Na+.(2) Ouabain or iodoacetate plus cyanide exerted an inhibitory effect on the outfiux but not the influx of Na+.(3) When K+ was added to a medium containing particles loaded with Na+ (22Na), an immediate release of Na+ from these particles was observed; this suggests the existence of a Na+–K+ exchange transport system.(4) The K+ effect was inhibited by 10‐4 M‐ouabain only at low (about 3·3 mm), but not at high (20 mm), K+ concentrations.(5) The uptake and release of Na+ by the nerve‐ending particles were found to be temperature‐dependent.(6) Only nerve‐ending particles with intact synaptosomal membranes were found to transport Na+ actively.