The phenomenon of antidromic inhibition in the spinal cord is generally considered to depend on a population of intercalated interneurons which are cholinergically activated by motoneuron collaterals and are, in turn, responsible for the powerful IPSP''s generated in adjacent motoneurons and the coincident burst of rapid repetitive neural activity in the immediate surround. Detailed analysis of the structural substrate in the anterior horn using Golgi impregnations and related methods, does not support the circuit arrangements currently postulated to account for this effect. The problems inherent in the present model are considered in detail and an alternative substrate is suggested.