Intracellular recordings were made from identified bulbar reticulo‐spinal neurones in the medulla of lamprey ammocoetes. Responses to iontophoretically applied inhibitory transmitters were measured as changes in membrane potential and input resistance. Dose‐dependent alterations in the responses to γ‐aminobutyric acid (GABA) and glycine during bath application of injectable anaesthetic drugs were measured; the compounds used were pentobarbitone, ketamine, metomidate and the steroid mixture alphaxalone/alphadolone (Saffan). GABA responses were potentiated by pentobarbitone (1–3 × 10−4m) and prolonged by ketamine (3.7 × 10−5m); but depressed by high concentrations (10−3m) of all drugs, as well as by anaesthetic concentrations of alphaxalone (1–3 × 10−5m). Glycine responses were depressed by alphaxalone (1–3 × 10−5m) and by supra‐anaesthetic concentrations of ketamine (3.7 × 10−4 m) and metomidate (1.8 × 10−3m). No drug potentiated the glycine responses. In the absence of an effect common to the 4 anaesthetics, it is concluded that neither potentiation nor inhibition of all GABA or glycine responses is an essential feature of anaesthesia. However, effects comparable to those described here may contribute to the overall clinical picture during anaesthesia of higher vertebrates. The findings do not support the notion that all anaesthetic agents act on biological membranes by a single mechanism.

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