The actions of 13 general anaesthetics (diethyl ether, enflurane, isoflurane, methoxyflurane, sevoflurane, chloral hydrate, trifluoroethanol, tribromoethanol, tert-butanol, chloretone, brometone, trichloroethylene, and alpha-chloralose) were studied on agonist-activated Cl(-) currents at human GABA(A) alpha(2)beta(1), glycine alpha(1), and GABA(C) rho(1) receptors expressed in human embryonic kidney 293 cells. All 13 anaesthetics enhanced responses to submaximal (EC(20)) concentrations of agonist at GABA(A) and glycine receptors, except alpha-chloralose, which did not enhance responses at the glycine alpha(1) receptor. None of the anaesthetics studied potentiated GABA responses at the GABA(C) rho(1) receptor. Potentiation of submaximal agonist currents by the anaesthetics was studied at GABA(A) and glycine receptors harbouring mutations in putative transmembrane domains 2 and 3 within GABA(A) alpha(2), beta(1), or glycine alpha(1) receptor subunits: GABA(A) alpha(2)(S270I)beta(1), alpha(2)(A291W)beta(1), alpha(2)beta(1)(S265I), and alpha(2)beta(1)(M286W); glycine alpha(1)(S267I) and alpha(1)(A288W). For all anaesthetics studied except alpha-chloralose, at least one of the mutations above abolished drug potentiation of agonist responses at GABA(A) and glycine receptors. alpha-Chloralose produced efficacious direct activation of the GABA(A) alpha(2)beta(1) receptor (a 'GABA-mimetic' effect). The other 12 anaesthetics produced minimal or no direct activation of GABA(A) and glycine receptors. A non-anaesthetic isomer of alpha-chloralose, beta-chloralose, was inactive at GABA(A) and glycine receptors and did not antagonize the actions of alpha-chloralose at GABA(A) receptors. The implications of these findings for the molecular mechanisms of action of general anaesthetics at GABA(A) and glycine receptors are discussed.