Several ionotropic receptors activated by ambient neurotransmitters have been reported to modulate axonal excitability and synaptic transmission in the cortex. These include extrasynaptic and presynaptic nicotinic, GABA_A (Ruiz et al. 2003), kainate, AMPA and NMDA receptors. We have examined the properties of axonal GABA_A receptors in rat hippocampal mossy fibres by studying changes in the threshold for antidromic action potential activation, recorded via somatic voltage clamp. Antidromic action currents were elicited in dentate granule cells by positioning a stimulating electrode in stratum lucidum. Application of the endogenous neurosteroid tetrahydrodeoxycorticosterone (THDOC, 10 nM) increased the threshold for antidromic action potential activation (n = 7; p < 0.01, t test) arguing for a role of δ subunit-containing GABA_A receptors at mossy fibres. Conversely, the GABA_A receptor blocker bicuculline (10 μM) lowered the threshold. Furthermore, the GABA receptor agonist muscimol (10 μM) evoked single channel openings in outside-out patches taken from mossy fibre boutons directly demonstrating axonal GABA_A receptors. The single channel conductance was estimated between 35 and 43 pS in different patches (n = 3). The THDOC sensitivity suggests that mossy fibres express tonically active GABA_A receptors with δ containing subunits. However, γ subunit-containing receptors may coexist.