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Insect epithelia most commonly absorb from KCl-rich, low Na+ fluids. This is true of the locust hindgut, which is functionally analogous to vertebrate kidney tubules. Active absorption of Cl- at the apical membrane is the predominant transport process giving rise to a large short-circuit current (I-sc) after stimulation by neuropeptides (CTSH, ITP) via cAMP as second messenger. This Cl- transport is not coupled to or driven secondarily by Na+, K+, HCO3-, Ca2+, or Mg2+ movements. An apical V-type H+ ATPase acidifies the hindgut lumen but at a rate that is 10-15% of Cl-dependent I-sc. The evidence to date as to whether the resulting large apical proton gradient is used to drive Cl- transport secondarily by an apical H+/Cl- symport is mixed. Thus a primary mechanism of Cl- absorption remains an alternative possibility. The complete primary structure of a large neuropeptide stimulant (ITP: 72 amino acids) of locust ileal Cl- transport has recently been deduced from its cDNA. This is the first putative insect neuropeptide hormone shown to stimulate ion transport across absorptive epithelia for which the primary sequence has been deduced. (C) 1996 Wiley-Liss, Inc.