Single channel recordings were obtained from a Drosophila S2 cell line stably expressing the wild-type RDL(ac) Drosophila melanogaster homomer-forming ionotropic GABA receptor subunit, a product of the resistance to dieldrin gene, RDL: GABA (50 microM) was applied by pressure ejection to outside-out patches from S2-RDL cells at a holding potential of -60 mV. The resulting inward current was completely blocked by 100 microM picrotoxin (PTX). The unitary current-voltage relationship was linear at negative potentials but showed slight inward rectification at potentials more positive than 0 mV. The reversal potential of the current (E(GABA)=-1.4 mV) was close to the calculated chloride equilibrium potential. The single channel conductance elicited by GABA was 36 pS. A 71 pS conductance channel was also observed when the duration of the pulse, used to eject GABA, was longer than 80 ms. The mean open time distribution of the unitary events was fitted best by two exponential functions suggesting two open channel states. When either 1 microM fipronil or 1 microM BIDN was present in the external saline, the GABA-gated channels were completely blocked. When BIDN or fipronil was applied at a concentration close to the IC(50) value for suppression of open probability (281 nM, BIDN; 240 nM, fipronil), the duration of channel openings was shortened. In addition, the blocking action of BIDN resulted in the appearance of a novel channel conductance (17 pS). The effects of co-application of BIDN and fipronil were examined. Co-application of BIDN (300 nM) with various concentrations (100-1000 nM) of fipronil resulted in an additional BIDN-induced dose-dependent reduction of the maximum P(o) value. Thus both BIDN and fipronil shorten the duration of wild-type RDL(ac) GABA receptor channel openings but appear to act at distinct sites.