Cl- current in GABA-sensitive neurons of the frog dorsal root ganglia was separated from other Na+, Ca2+ and K+ currents using the suction pipette technique which allows internal perfusion and current clamp. Adequacy of the internal perfusion technique was assessed from the reversal potential for GABA-induced Cl- current (EGABA) at various intracellular Cl- concentrations. EGABA was equal to the Cl- equilibrium potential (ECl) and behaved as a simple Cl- electrode following changes of external and internal Cl- concentrations [(Cl]0 and [Cl]i. EGABA changed by 58 mV for a tenfold change in either [Cl]0 or [Cl]i at constant [Cl]0 or [Cl]i, respectively. GABA-induced Cl- conductance increased dose-dependently. In addition, single Cl- channel current (icl) and conductance (gamma Cl) were estimated from the Cl- current fluctuation by using both power spectrum and amplitude histogram methods, and the theoretical values were compared with those measured directly by a conventional inside-out patch clamp technique.