The peptide hormone neurotensin (NT) is found mainly in gut endocrine cells of the ileum, but has also been identified as a putative neurotransmitter in the central and peripheral nervous systems. It may have a dual role as a circulating gastrointestinal hormone and peripheral neurotransmitter. Its predominant effects are to reduce oesophageal sphincter tone, inhibit gastric secretion and emptying and inhibit intestinal motility, but stimulate intestinal and pancreatic exocrine secretion; NT-like immunoreactivity has been found in kidney and therefore NT may influence renal function. When infused i.v. in rabbits it causes antinatriuresis. We have studied its renal effects in 11 healthy males by i.v. infusion under conditions of altered dietary sodium. Postprandial circulating neurotensin levels were reproduced by infusion. There were no consistent systemic or renal haemodynamic effects. Plasma electrolytes and renin did not change. Only renal chloride excretion changed significantly, falling by ca. 30%, and recovering after infusion. There is no evidence for a specific renal tubular chloride transport mechanism, but coupled cotransport, Na+:K+:2CI-, may be hormonally regulated. NT might stimulate this process and contribute to the renal response to changes in dietary composition, especially sodium intake.