A longstanding question about the possible dependence of transmembrane peptide transport on sodium has now been resolved. Recent studies with purified intestinal brush border membrane vesicles have shown that peptide transport across this membrane is Na+-independent and occurs by a non-concentrative mechanism. Similar studies with renal brush border membrane vesicles have established for the first time the presence of a peptide transport system in mammalian kidney. The essential characteristics of peptide transport in these two tissues are the same. However, it still remains to be seen whether a new mechanism other than the Na+-gradient, hitherto unrecognized, is involved in energizing the active transport of peptides in vivo in mammalian intestine and kidney.