Freshly dispersed sheep mesenteric lymphatic smooth muscle cells were studied at 37°C using the perforated patch-clamp technique with Cs⁺- and K⁺-filled pipettes. Depolarizing steps evoked currents that consisted ofl-type Ca²⁺ [I _Ca(L)] current and a slowly developing current. The slow current reversed at 1 ± 1.5 mV with symmetrical Cl⁻ concentrations compared with 23.2 ± 1.2 mV (n = 5) and −34.3 ± 3.5 mV (n = 4) when external Cl⁻ was substituted with either glutamate (86 mM) or I⁻ (125 mM). Nifedipine (1 μM) blocked and BAY K 8644 enhanced I _Ca(L), the slow-developing sustained current, and the tail current. The Cl⁻ channel blocker anthracene-9-carboxylic acid (9-AC) reduced only the slowly developing inward and tail currents. Application of caffeine (10 mM) to voltage-clamped cells evoked currents that reversed close to the Cl⁻ equilibrium potential and were sensitive to 9-AC. Small spontaneous transient depolarizations and larger action potentials were observed in current clamp, and these were blocked by 9-AC. Evoked action potentials were triphasic and had a prominent plateau phase that was selectively blocked by 9-AC. Similarly, fluid output was reduced by 9-AC in doubly cannulated segments of spontaneously pumping sheep lymphatics, suggesting that the Ca²⁺-activated Cl⁻ current plays an important role in the electrical activity underlying spontaneous activity in this tissue.