The patch clamp technique has been used to study ion channel activity in the apical (ventricular) membrane of epithelial cells from the rat choroid plexus. Two different classes of Cl(-)-selective channel were identified. A low conductance (26 pS) channel which was the predominant feature in cell-attached and inside-out patches. The occurrence of this channel appeared to increase in tissue bathed in forskolin. It was activated in inside-out patches by increasing the Ca2+ concentration at the intracellular face of the membrane and by depolarising potentials. The second class of channel was observed infrequently (2% of patches) and appeared to be similar to 'maxi'-Cl- channels which have been described in many other cell types. It had a conductance of 320 pS, opened to sub-conductance levels and displayed a marked voltage dependence in inside-out patches. The possible contribution of these channels to Cl- transport during the production of cerebrospinal fluid (CSF) is discussed.