We have sought evidence for intrinsic oscillatory membrane properties in spinal cord neurons which participate in fictive locomotion in Rana temporaria embryos. In the presence of tetrodotoxin to synaptically isolate intracellularly recorded neurons, the bath application of N-methyl-d-aspartate (NMDA) depolarizes neurons by 20 to 30 mV. It does not, however, elicit continuous membrane potential oscillations, as have been found in homologous spinal neurons of the lamprey. However, addition of the neuromodulatory amine 5-HT to the bathing medium rapidly induces repetitive large scale (up to 40mV) oscillations in membrane potential. These oscillations depend upon the presence of magnesium ions and are abolished by the NMDA antagonist, APV. They do not occur when 5-HT is applied in the absence of NMDA. Expression of oscillatory membrane behaviour thus appears to rely upon an interaction between 5-HT and NMDA receptors. Neuroanatomical evidence shows that endogenous 5-HT-containing descending projections are present in the ventrolateral margins of the spinal cord at this stage in development and could therefore trigger these oscillations during normal motor behaviour. Preliminary pharmacological studies support the conclusion that activation of 5-HT1a receptors is involved in the induction process.