Gaboxadol (THIP; 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) is an agonist at GABA(A) receptors. THIP concentrations (0.01-50 mM) were applied rapidly to Sf9 cells expressing the human alpha(1)beta(2)gamma(2S) GABA(A) receptors. The EC(50) values for the peak current in THIP alone or THIP plus 1 microM diazepam were 154 and 53 microM, respectively. In supersaturating THIP (10-50 mM) the rate of current decay increased and an off-current developed when THIP was rapidly removed. The mean currents measured over the first 4 s in 10 mM and higher THIP concentrations were 0.6 or less of the 1 mM THIP mean current. Diazepam (1 microM) increased the 4 s mean current when evoked by 10 to 20 mM THIP but not 50 mM THIP. No similar effects on the current time-course were recorded in supersaturating gamma-aminobutyric acid (GABA) concentrations (50 and 80 mM). The results demonstrate an inhibitory as well as agonist effect of THIP at alpha(1)beta(2)gamma(2S) GABA(A) receptors.