SUMMARY1. A voltage-clamp study was made of some properties of the non-synaptic hyperpolarization-activated C1-conductance recently described in Aplysia neurones loaded with Cl-ions (Chesnoy-Marchais, 1982). The experiments were performed on an identified family of neurones, which present cholinergic responses allowing an easy measurement of the equilibrium potentials of Cl-(EC1) and K+ ions (EK).2. The Cl-selectivity ofthe hyperpolarization-activated conductance was deduced from four observations: (1) the extrapolated reversal potential of the hyperpolarization-activated current, Er, was close to the reversal potential ofthe cholinergic Cl-response, which is the equilibrium potential for Cl-ions, EC1. 3. The steady-state Cl-conductance (ass) increases steeply with hyperpolarization.The kinetics of activation and deactivation are exponential and are characterized by the same voltage-dependent time constant (r), of the order of a few seconds or fractions of seconds. The curves g.8(V) and r( V) can both be fitted by a two-state model in which the rate constants are exponential functions of the membrane potential (e-fold change for 12-16 mV).4. The Cl-current is much more affected by changes of the intracellular Clconcentration than predicted simply from the change in Cl-driving force. Both the conductance and the time constant of activation are strongly modified. Modifications of the extracellular Cl-concentration do not always alter the amplitude of the hyperpolarization-activated Cl-current, but systematically affect its kinetics.5. The hyperpolarization-activated current is abolished after prolonged exposure of the cell to an artificial sea water where N03-ions replace Cl-ions, as well as after intracellular injections of N03-ions.6. Increasing the external pH shifts the g.(V) and r(V) curves to the left.Lowering the external pH has reverse but less pronounced effects. 7. In cells which were not loaded with Cl-ions and did not present the hyperpolarization-activated Cl-current, this current could be detected if the D. CHESNO Y-MARCHAIS hyperpolarizing jump was preceded by short depolarizing pulses. In cells which were loaded with Cl-ions, the Cl-current became larger after a short depolarizing pulse.In the presence of extracellular C02+ ions, depolarizing pulses no longer increased the Cl-current.8. The Cl-current is not affected by extracellularly applied DIDS (4,4'-diisothiocyano-2,2'-disulphonic acid stilbene), but is markedly reduced by intracellular injection of DIDS.9. Extracellular Cs+ ions, which have been reported to block some cationic hyperpolarization-activated inward currents, do not reduce the hyperpolarizationactivated Cl-current. High concentrations of Cs+ produce complex effects which are probably due to an increased synaptic activity, but the hyperpolarization-activatedCl-current persists after complete substitution of the extracellular and intracellular monovalent cations by Cs+.