In rat kidney the “secretory” isoform of the Na⁺-K⁺-2Cl⁻ cotransporter (NKCC1) localizes to the basolateral membrane of the α-intercalated cell. The purpose of this study was to determine whether rat outer medullary collecting duct (OMCD) secretes Cl⁻ and whether transepithelial Cl⁻ transport occurs, in part, through Cl⁻ uptake across the basolateral membrane mediated by NKCC1 in series with Cl⁻ efflux across the apical membrane. OMCD tubules from rats treated with deoxycorticosterone pivalate were perfused in vitro in symmetrical HCO/CO₂-buffered solutions. Cl⁻ secretion was observed in this segment, accompanied by a lumen positive transepithelial potential. Bumetanide (100 μM), when added to the bath, reduced Cl⁻ secretion by 78%, although the lumen positive transepithelial potential and fluid flux were unchanged. Bumetanide-sensitive Cl⁻ secretion was dependent on extracellular Na⁺ and either K⁺ or NH, consistent with the ion dependency of NKCC1-mediated Cl⁻ transport. In conclusion, OMCD tubules from deoxycorticosterone pivalate-treated rats secrete Cl⁻into the luminal fluid through NKCC1-mediated Cl⁻ uptake across the basolateral membrane in series with Cl⁻ efflux across the apical membrane. The physiological role of NKCC1-mediated Cl⁻ uptake remains to be determined. However, the role of NKCC1 in the process of fluid secretion could not be demonstrated.