Most barbiturates are anaesthetics but a few unexpectedly are convulsants. We recently located the anaesthetic sites on GABAARs by photolabeling with an anaesthetic barbiturate. To apply the same strategy to locate the convulsant sites requires the creation and mechanistic characterization of a suitable agent. We synthesized enantiomers of a novel, photoactivable barbiturate, mTFD‐MPPB (1‐methyl‐5‐propyly‐5‐(m‐trifluoromethyldiazirinyl) phenyl barbituric acid). In mice, S–mTFD‐MPPB acted as a convulsant, whereas R–mTFD‐MPPB acted as an anticonvulsant. Using patch clamp electrophysiology and fast solution exchange on recombinant human α1β3γ2L GABAARs expressed in HEK cells, we found that S–mTFD‐MPPB inhibited GABA–induced currents, whereas R–mTFD‐MPPB enhanced them. S–mTFD‐MPPB caused inhibition by binding to either of two inhibitory sites on open channels with bimolecular kinetics. It also inhibited closed, resting state receptors at similar concentrations, decreasing the channel opening rate and shifting the GABA concentration‐response curve to the right. R–mTFD‐MPPB, like most anaesthetics, enhanced receptor gating by rapidly binding to allosteric sites on open channels, initiating a rate‐limiting conformation change to stabilized open channel states. These states had slower closing rates, thus shifting the GABA concentration‐response curve to the left. Under conditions when most GABAARs were open, an inhibitory action of R–mTFD‐MPPB was revealed that had a similar IC50 to S–mTFD‐MPPB's. Thus, the inhibitory sites are not enantioselective, and the convulsant action of S–mTFD‐MPPB results from its negligible affinity for the enhancing, anaesthetic sites. Interactions with these two classes of barbiturate–binding sites on GABAARs underlie the enantiomers’ different pharmacological activities in mice. This article is protected by copyright. All rights reserved