OSMOREGULATION, inhibitory neurotransmission and pH balance depend on chloride ion (Cl-) flux. In intestinal epithelial cells, apical Cl- channels control salt and fluid secretion and are, in turn, regulated by agonists acting through cyclic nucleotides and internal calcium ion concentration ([Ca2+](i))(1-3). Recently, we found that muscarinic pretreatment prevents [Ca2+](i) increases from eliciting Cl- secretion in T-84 colonic epithelial cells(4). By studying concomitant inositol phosphate metabolism, ae have now identified D-myo-inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P-4), as the inositol phosphate most likely to mediate this uncoupling. A novel, membrane-permeant ester prepared by total synthesis delivers Ins(3,4,5,6)P-4 intracellularly and confirms that this emerging messenger(5) does inhibit Cl- flux resulting from thapsigargin- or histamine-induced [Ca2+](i) elevations.