A two-microelectrode current-voltage clamp and Cl(-)-selective microelectrodes were used to examine the effects of gamma-aminobutyric acid (GABA) on membrane potential, current and intracellular Cl- activity (aiCl) in the crayfish stretch receptor neurone. All experimental solutions were CO2-HCO3- free. 2. GABA (500 microM) produced a mono- or biphasic depolarization (amplitude < or = 10 mV), often with a prominent initial depolarizing component followed by a transient shift to a more negative level. In some neurones, an additional depolarizing phase was seen upon washout of GABA. Receptor desensitization, being absent, played no role in the multiphasic actions of GABA. 3. The pronounced increase in membrane conductance evoked by GABA (500 microM) was associated with an increase in aiCl which indicates that the depolarizing action was not due to a current carried by Cl- ions. 4. The currents activated by GABA under voltage clamp conditions were inwardly directed when recorded at the level of the resting membrane potential, and they often revealed a biphasic character. The reversal potential of peak currents activated by pulses of 500 microM-GABA (EGABA) was 9-12 mV more positive than the reversal potential of the simultaneously measured net Cl- flux (ECl). ECl was 2-7 mV more negative than the resting membrane potential. 5. EGABA (measured using pulses of 500 microM-GABA) was about 10 mV more positive than the reversal potential of the current activated by 500 microM-muscimol, a GABA agonist that is a poor substrate of the Na(+)-dependent GABA uptake system. 6. In the absence of Na+, the depolarization and inward current caused by 500 microM-GABA were converted to a hyperpolarization and to an outward current. Muscimol produced an immediate outward current both in the presence and absence of Na+. 7. Following block of the inhibitory channels by picrotoxin (100-200 microM), the depolarizing effect of 500 microM-GABA was enhanced and the transient hyperpolarizing shifts were abolished. 8. In the presence of picrotoxin, GABA (> or = 2 microM) produced a concentration-dependent monophasic inward current which had a reversal potential of +30 to +60 mV. This current was inhibited in the absence of Na+ and by the GABA uptake blocker, nipecotic acid. Unlike the channel-mediated current, the picrotoxin-insensitive current was activated without delay also at low (2-10 microM) concentrations of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)