Abstract In the isolated ileum of the guinea-pig, γ-aminobutyric acid (GABA) elicited both GABA A- and GABA B-receptor-mediated responses. Barbiturates significantly potentiated the GABA A receptor-induce contration over the lower dose range of applied GABA (< 50 μM), without affecting the GABA B-receptor-induced ‘after-relaxation’, their relative potencies being thiopentone (ThP) > pentobarbitone (PB) > barbitone (Bb) > phenobarbitone (PhB). Also, higher doses of ThP, PB and Bb elicited bicuculline- and pictrotoxinin-sensitive GABA-like contrastions. PB shifted the dose-response curve for GABA in the presence of bicuculline methochloride to the left without altering the slope or the dose ratio, and not only displaced the dose-response curve for GABA in the presence of picrotoxinin but also restored the slope and the maximum response towards that of the control GABA-induced response. These results further characterise the GABA A-receptor sites of the guinea-pig enteric nervous system, where GABA is evidently a neurotransmitter.