Whole-cell patch clamp experiments were performed on cultured human cytotrophoblast cells incubated for 24-48 hr after their isolation from term placentas. Cl--selective currents were examined using K+-free solutions. Under nonstimulated conditions, most cells initially expressed only small background leak currents. However, inclusion of 0.2 mM GTPgammaS in the electrode solution caused activation of an outwardly rectifying conductance which showed marked time-dependent activation at depolarized potentials above +20 mV. Stimulation of this conductance by GTPgammaS was found to be Ca2+-dependent since GTPgammaS failed to activate currents when included in a Ca2+-free electrode solution. In addition, similar currents could be activated by increasing the [Ca2+] of the pipette solution to 500 nM. The Ca2+-activated conductance was judged to be Cl--selective, since reversal potentials were predicted by Nernst equilibrium potentials for Cl-. This conductance could also be reversibly inhibited by addition of the anion channel blocker DIDS to the bath solution at a dose of 100 microM. Preliminary experiments indicated the presence of a second whole-cell anion conductance in human cytotrophoblast cells, which may be activated by cell swelling. Possible roles for the Ca2+-activated Cl- conductance in human placental trophoblast are discussed.