The patch-clamp technique was used to study the effects of carbachol (CCh) on HT-29 cells. During CCh exposure, the cells (n = 23) depolarized close to the equilibrium potential for Cl (; 48 mV) and the membrane potential then started to oscillate (16/23 cells). In voltage-clamp experiments, similar oscillations in whole cell currents could be demonstrated. The whole cell conductance increased from 225 ± 25 pS in control solution to 6,728 ± 1,165 pS (means ± SE, n = 17). In substitution experiments (22 mM Cl in bath solution,  = 0 mV), the reversal potential changed from 41.6 ± 2.2 mV (means ± SE, n = 9) to 3.2 ± 2.0 mV (means ± SE, n = 7). When the cells were loaded with the calcium-sensitive fluorescent dye, fluo 3, and simultaneously patch clamped, CCh caused a synchronous oscillating pattern of fluorescence and membrane potential. In cell-attached patches, the CCh-activated currents reversed at a relative membrane potential of 1.9 ± 3.7 mV (means ± SE, n = 11) with control solution in the pipette and at 46.2 ± 5.3 mV (means ± SE, n = 10) with a 15 mM Cl solution in the pipette. High K⁺ (144 mM) did not change the reversal potential significantly (P  0.05, n = 8). In inside-out patches, calcium-dependent Cl channels could be demonstrated with a conductance of 19 pS (n = 7). It is concluded that CCh causes oscillations in membrane potential that involve calcium-dependent Cl channels and a K⁺ permeability.