Field potentials recorded from a group of cells in the intermediomedial gray matter of spinal cord slices from neonatal rats showed long-term potentiation and long-term depression in response to brief bursts of high frequency conditioning stimulation. Both N-methyl-D-aspartic acid-dependent and N-methyl-D-aspartic acid-independent long-term potentiation was seen. The long-term depression was partially reversed, but not prevented from occurring, by the opiate antagonist naloxone. The frequency of occurrence of long-term potentiation and depression was affected by the inhibitory blockers bicuculline and strychnine in a fashion consistent with the hypothesis that a slight depolarization favoured long-term potentiation and a larger depolarization favoured long-term depression.