The properties of the GABA-activated ion channel in hippocampal neurones prepared from 17- to 19-day-old fetal rats in dispersed cell cultures were studied with the patch-clamp techniques. These neurones had chemosensitivity to gamma-aminobutyric acid (GABA) in the absence of synaptic inputs. GABA activated chloride ion channels selectively in these neurones. The GABA-induced Cl- current was detectable with greater than 1 microM GABA. The amplitude of the current increased with higher concentrations of GABA and apparently saturated at 100 microM. The effective single-channel conductance (gamma) was estimated to be 19 pS from the mean and variance of the steady-state GABA-induced current fluctuation in the whole-cell recordings. The power density spectra for GABA-induced current fluctuations in the whole-cell as well as cell-free outside-out membrane patch recordings had more than a single Lorentzian component. The application of GABA induced discrete pulse-like current flows through the cell-free outside-out membrane patch, after the number of channels activated by GABA had decreased due to the rundown. The single-channel conductance estimated from the amplitude of the current pulse was 29 pS when the intra- and extracellular Cl- concentrations were 150 and 120 mM, respectively. In addition to the above conducting state, the GABA channel had several open states with lower conductances. The apparent discrepancy between the effective single-channel conductance estimated from noise analysis and the single-channel conductances directly measured with individual channels may be due, at least in part, to the presence of multiple conducting states.