1. The nature of Cl- conductance(s) participating in transepithelial anion secretion by renal inner medullary collecting duct (IMCD, mIMCD-K2 cell line) was investigated. 2. Extracellular ATP (100 microM) stimulated a transient increase in both whole-cell Cl- conductance and intracellular free Ca2+. In contrast, ionomycin (10-100 nM) caused a sustained increase in whole-cell Cl- conductance. Pre-loading cells with the Ca2+ buffer BAPTA abolished the ATP-dependent responses and delayed the onset of the increase observed with ionomycin. 3. The Ca2+-activated whole-cell Cl- current stimulated by ATP (peak) and ionomycin (maximal) displayed (i) a linear steady-state current-voltage relationship and (ii) time and voltage dependence with slow activation at +80 mV and slow inactivation at -80 mV. In BAPTA-loaded cells, ionomycin-elicited whole-cell currents exhibited pronounced outward rectification with time-dependent activation/inactivation. 4. Ca2+-activated and forskolin-activated Cl- conductances co-exist since ATP activation of whole-cell current occurred during a maximal stimulation by forskolin in single cell recordings. 5. In IMCD epithelial layers, ATP and ionomycin stimulated an inward short circuit current (Isc) dependent upon basal medium Na+ and Cl-/HCO3- but independent of the presence of apical bathing medium Na+ and Cl-/HCO3-. This was identical to forskolin stimulation and consistent with transepithelial anion secretion. 6. PCR amplification of reverse-transcribed mRNA using gene-specific primers demonstrated expression of both cystic fibrosis transmembrane conductance regulator (CFTR) mRNA and Ca2+-activated Cl- channel (mCLCA1) mRNA in mIMCD-K2 cells. 7. Ca2+ and forskolin-activated Cl- conductances participate in anion secretion by IMCD.