Recent morphological, physiological, and molecular studies that shed light on the functional significance of the neurohypophysial peptide arginine vasotocin (AVT) in the reptilian kidney are reviewed. Several structural features of the reptilian nephron are highlighted, including the presence of an incipient juxta-glomerular apparatus, aglomerular nephrons, and the thin, very short intermediate segment (IS) joining proximal and distal convoluted tubules. Although the V-2-like AVT receptor and its gene have yet to be sequenced in any reptile, AVT receptors have been located in both the IS and the branched collecting duct system in the agamid lizard Ctenophorus ornatus. These findings suggest that AVT may have a dual effect in the reptilian kidney, first diluting the urinary fluid in the thin-intermediate segment, prior to its entering the collecting duct system, and then facilitating water reabsorption along an osmotic gradient as the urine passes through the final segments of the nephron. The IS may thus prove to be the evolutionary homologue of the thin-ascending limb of the loop of Henle in the mammalian kidney. (C) 2002 Elsevier Science (USA).