We studied the combined effect of diazepam and GABAA-ergic ligands on the activity of Cl–-ATPase in plasma membrane of bream brain. The membrane fraction were preincubated and incubated with diazepam as well as with other GABAA-ergic ligands at physiological pH (7.4), i.e. under the conditions when Cl–-ATPase activity is undetectable. GABA (0.1–100 μM) induced Cl–-ATPase activity with the maximum effect at 10 μM. Diazepam (0.1 μM) enhanced the effect of low GABA concentrations (0.1–1 μM) on Cl–-ATPase activity but had no effect on the enzyme in the presence of high GABA concentrations (10–100 μM). At the same time, GABA (1 μM) enhanced the effect of low diazepam concentrations (0.1–1 μM) on the enzyme activity but had no effect on it in the presence of high concentrations of the ligand. Blockers of GABAA-ergic receptors, picrotoxin (50 μM) and bicuculline (5 μM), canceled the combined effect of diazepam and GABA on the enzyme activity. The obtained data demonstrate that the combined effect of diazepam and GABAA-ergic ligands on Cl–-ATPase activity at physiological pH is similar to the effect of these ligands on GABAA/benzodiazepine/Cl– channel.