The action of tetanus toxin on spinal reflexes was described by Sherrington (1905) as a conversion of inhibition into excitation. In general the clinical effects produced by the actions of tetanus toxin and of strychnine are very similar, and because of this Sherrington (1905, 1906) suggested that it was likely that these two substances had the same action in the central nervous system. Sherrington's observations were based upon experiments in which the inhibition of extensor reflexes was replaced by facilitation, the con-ditioning volleys having beenset up in theinternal saphenous or hamstring nerves.

When strychnine is injected intravenously in subconvulsive doses it greatly diminishes the amount of inhibition producedby a'direct'inhibitory volley, but has no significant effect on the testing monosynaptic reflex (Bradley, Easton & Eccles, 1953). Intracellular records from motoneurones reveal that strychnine diminishes inhibitory action by depressing the inhibitory post-synaptic potential (IPSP) produced by impulses in inhibitory presynaptic fibres (Coombs, Eccles & Fatt, 1955). In view of thesimilarity of the central effects of strychnine and of tetanus toxin it was thought of interest to investigate the effect of tetanus toxin upon spinal inhibitory mechanisms. Five main types of inhibition have been investi-gated: the'direct'inhibition of motoneurones by impulses in the group Ia afferent fibres of antagonistic muscles; the inhibition by impulses in the group Ib afferent fibres from muscles of the same limb; the inhibition of extensor motoneurones by impulses in the groups II and III muscle afferent fibres and in cutaneous afferent fibres; and the inhibition of motoneurones