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Department of Pharmacology and Clinical Pharmacology, St George's Hospital Medical School, London.

1. Membrane responses were recorded using a standard patch pipette technique in isolated cells freshly dispersed from the rat anococcygeus muscle. Using the whole-cell mode of recording, the ionophoretic application of noradrenaline evoked depolarization (current clamp) and inward current (voltage clamp) at membrane potentials of -50 to -60 mV. 2. The inward current to noradrenaline was reversibly blocked by low concentrations of phentolamine indicating that the response is mediated by alpha-adrenoceptors. The inward current was associated with a conductance increase. 3. The ionic mechanism of the inward current was investigated in potassium-free solutions. Alteration of the chloride equilibrium potential produced similar changes in the reversal potential of the noradrenaline-induced current and it is concluded that noradrenaline opens chloride-selective channels. 4. The noradrenaline-activated chord conductance was increased by depolarization. From a holding potential of -50 mV depolarizing voltage steps in the presence of noradrenaline produced relaxations and the time constants of these relaxations at -50 and +30 mV were respectively 89 and 166 ms. 5. The calcium ionophore A23187 and caffeine applied in the bathing solution also induced an increase in chloride conductance. 6. It is concluded that in isolated cells of the rat anococcygeus muscle the depolarization to noradrenaline is mediated by an increase in membrane chloride conductance.

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