The positive allosteric effects of four structurally distinct general anaesthetics (propofol, pentobarbitone, etomidate and 5α-pregnan-3α-ol-20-one [5α3α]) upon recombinant GABA(A) (α6β3γ(2L)), invertebrate GABA (RDL) and glycine (α1) receptors expressed in Xenopus laevis oocytes have been determined. Propofol and pentobarbitone enhanced agonist (GABA or glycine as appropriate)-evoked currents at GABA(A), glycine, and RDL receptors, whereas etomidate and 5α3α were highly selective for the GABA(A) receptor. Utilizing site-directed mutagenesis, we demonstrate that the nature of the interaction of propofol, pentobarbitone and etomidate (but not 5α3α) with mammalian and invertebrate ionotropic GABA receptors depends critically upon the nature of a single amino acid located in the second transmembrane region (TM2) of these receptors. These data are discussed in relation to the specificity of action of general anaesthetics. Copyright (C) 1999.