Ventral roots of the newborn rat spinal cord were stimulated while recording intracellularly from motoneurons. In many cells, stimulation subthreshold for an antidromic action potential in the impaled cell produced a small, short-latency depolarization, which was unaffected by membrane polarization. This response (antidromic synaptic potential, a.s.p.) was also seen, in some cells, on stimulating the ventral root of an adjacent segment. Replacement of Ca2+ (2 mM) with Mn2+ (3 mM) or Mg2+ (10 mM) completely abolished orthodromic synaptic potentials, but the a.s.p. persisted. These results strongly suggest that the a.s.p. is produced by an electrical interaction between motoneurons.