AbstractThe effect of seven intermediates of the TCA cycle, of pyruvate and glutamate, was studied on the impulse activity and oxygen uptake of the slowly adapting cell of the stretch receptor organ of crayfish. Citrate, isocitrate, α‐ketoglutarate and succinate showed a strong excitatory action, fumarate, malate, oxaloacetate and pyruvate had no action while glutamate had an inhibitory action on the impulse activity. The most effective substrate tested was citrate which also increased respiration sevenfold. Isocitrate showed a lower effect (2.2 fold) as did α‐ketoglutarate (4 fold), succinate (2.8 fold) and malate (3 fold). Fumarate, pyruvate and glutamate did not have any effect on the respiratory activity. The only substrate showing an inhibitory effect on oxygen uptake was oxaloacetate. Glucose, tested for comparison, also showed stimulatory effect (2.4 fold) on respiration. The authors are inclined to believe that the effect on the impusle activity and respiration exerted by these compounds is confined to the nerve cell membrane. The opposite modes of action of malate and oxaloacetate on respiration, which did not affect impulse activity, are however more difficult to explain in terms of a pure membrane mech