Microsomes formed from rabbit kidney medulla and reconstituted proteoliposomes formed from these microsomes were capable of amiloride-inhibited Na+ transport that was insensitive to valinomycin either with or without K+. This indicated that the Na+ transport process was electroneutral. This Na+ transport process was insensitive to extravesicular Cl- or HCO-3 and not stimulated by high intravesicular gradients of K+, Ca2+ or Mg2+, which indicated that the process did not require NaCl or NaHCO3 co-transport or Na+/K+, Na+/Ca2+ or Na+/Mg2+ counter-transport. Na+ uptake into microsomes or proteoliposomes was inhibited by extravesicular K+, Ca2+, Mg2+ or La3+, which indicated that these ions interacted with the Na+-binding site on the transport protein. Na+ uptake into microsomes was stimulated by intravesicular protons and inhibited by extravesicular protons. This suggested that microsomes were capable of Na+-H+ exchange and this was confirmed when Na+ was shown to stimulate H+ efflux from microsomes. The amiloride-inhibited Na+ transporter from medulla microsomes which has been reconstituted into proteoliposomes is most likely a Na+-H+ exchanger. This Na+ transporter was totally insensitive to the uncoupler 1799, either in the presence or absence of valinomycin plus K+ and less sensitive to NH3 than to amiloride. This indicated that amiloride inhibited Na+ transport not merely by acting as a weak-base uncoupler but by directly interacting with the protein responsible for Na+-H+ exchange.