1. The effects of gamma-aminobutyric acid (GABA) on membrane potential and conductance as well as on the intracellular Cl- activity (aiCl) and intracellular pH (pHi) were studied in crayfish muscle fibres using a three-microelectrode voltage clamp and ion-selective microelectrodes. In the presence of CO2-HCO3-, the intracellular HCO3- activity (aiHCO3) was estimated from pHi. 2. In a nominally HCO3(-)-free solution, a near-saturating concentration of GABA (0.2 mM) produced a marked increase in membrane conductance but little change in potential. In a solution containing 30 mM-HCO3- (equilibrated with 5% CO2 + 95% air; pH 7.4), the GABA-induced increase in conductance was associated with a depolarization of about 15 mV, with an increase in aiCl and with a decrease in aiHCO3. All these effects were blocked by picrotoxin (PTX). The depolarizing action of GABA was augmented following depletion of extracellular and intracellular Cl-. 3. The GABA-induced increase in aiCl which took place in the presence of HCO3- was blocked by clamping the membrane potential at its resting level. This indicates that the increase in aiCl was due to passive redistribution of Cl-. In both the presence and absence of HCO3-, the GABA-activated transmembrane flux of Cl- showed reversal at the level of the resting potential, which indicates that under steady-state conditions the Cl- equilibrium potential (ECl) is identical to the resting potential. 4. In a Cl(-)-free, 30 mM-HCO3(-)-containing solution, 0.5 mM-GABA produced a PTX-sensitive increase in conductance which amounted to 15% of the conductance activated in the presence of Cl-. In the absence of both Cl- and HCO3-, the respective figure was 2.8%. Assuming constant-field conditions, the conductance data yielded a permeability ratio PHCO3/PCl of 0.42 for the GABA-activated channels. 5. In a Cl(-)-containing, HCO3(-)-free solution, the reversal potential of the GABA-activated current (EGABA) was, by about 1 mV, less negative than the resting membrane potential (RP). In a solution containing Cl- and 30 mM-HCO3-, EGABA-RP was 12 mV. Simultaneous measurements of EGABA, aiCl and aiHCO3 (pHi) gave a PHCO3/PCl value of 0.33. 6. In a Cl(-)-free, HCO3(-)-containing solution EGABA was close to the HCO3- equilibrium potential (EHCO3) and an experimental acidosis which produced a negative shift in EHCO3 was associated with a similar shift in EGABA.(ABSTRACT TRUNCATED AT 400 WORDS)