The effect of some endogenous and synthetic steroids on the operation of inhibitory and excitatory amino acid neurotransmitter receptors was examined. Anaesthetic pregnane steroids (e.g. alphaxalone, 5 alpha-pregnan-3 alpha-ol-20-one, 5 alpha-pregnane-3 alpha,21-diol-20-one) potentiated GABAA receptor-mediated whole-cell currents recorded from bovine chromaffin cells. The threshold concentration for enhancement was 10-30 nM. Potentiation was stereoselective and was mediated by a steroid-induced prolongation of the burst duration of the GABA-activated channel. Additionally, the pregnane steroids directly activated the GABAA receptor. Both the potentiation and activation appear to be mediated through a site(s) distinct from the well-known barbiturate and benzodiazepine allosteric sites of the GABAA receptor. Intracellularly applied alphaxalone and 5 beta-pregnan-3 alpha-ol-20-one had no discernible effects on the GABAA receptor, suggesting that the steroid binding site can only be accessed extracellularly. Unlike behaviourally depressant barbiturates, which modulate GABAA receptor function in a manner similar to that of the pregnane steroids, alphaxalone and 5 beta-pregnan-3 alpha-ol-20-one show striking pharmacological selectivity. Voltage-clamp recordings from rat central neurons in culture indicate that pentobarbitone exerts its potentiating and GABA-mimetic effects over a range of concentrations which also depress currents mediated by glutamate receptor subtypes. In contrast, alphaxalone and several endogenous steroids greatly enhance responses to GABA, but have no direct effect on glutamate receptors. Such pharmacological selectivity, coupled with appropriate stereoselectivity of action, suggests that the GABAA receptor mediates some of the behavioural effects of synthetic and endogenous pregnane steroids.