1. The kinetic properties of the activation and inactivation (desensitization) phases of pentobarbitone (PB)-induced inward Cl- current (ICl) were studied in isolated frog sensory neurones, following suppression of Na+, K+ and Ca2+ currents, using the concentration jump technique which combines the internal perfusion and the rapid exchange of the external solutions surrounding a neurone with time constants of 2-3 ms. The results were compared with those of the gamma-aminobutyric acid (GABA)-gated ICl. 2. The PB dose-response curve was bell-shaped and the maximum peak value was less than the current induced by 1.7 X 1.5(-5) M-GABA, the concentration at which GABA evoked a half-maximum response. 3. The activation and inactivation phases of PB-induced ICl consisted of double-exponential, fast and slow components, respectively. The time constant of the fast component (tau af) of the activation was relatively stable in a concentration range between 3 X 10(-4) and 6 X 10(-3) M. The time constant of the slow component (tau as) of the activation decreased with increasing PB concentrations. Both the fast and slow components (tau if and tau is) of the inactivation decreased with increasing PB concentrations. 4. Over a wide range of concentrations the tau af and tau as values of the PB-induced ICl were 10-30 times greater than the respective values of GABA-induced ICl. 5. At concentrations below 10(-3) M the PB-induced ICl was voltage dependent at more negative potentials than -20 mV. 6. The PB-induced ICl was blocked by bicuculline and by picrotoxin, but in a different manner. Bicuculline increased the time constants of the activation and inactivation. Picrotoxin had little effect on the activation phase but markedly facilitated the inactivation phase. 7. High concentrations of PB (over 10(-3) M) led to a decline in both the peak and plateau currents of the PB-induced ICl. A transient 'hump' current appeared with wash-out of the external solutions containing high concentrations of PB. This hump current was blocked by bicuculline in a dose-dependent manner. 8. The results suggest the possibilities that the PB receptor-ionophore complexes consist of at least two different components having different affinities and kinetics and that the PB and GABA binding sites are closely located.