The swimming behaviour of lower vertebrates has been used as a model to study the function of simple neuronal circuits. Good examples are the lamprey and the Xenopus tadpole. In these two cases, glutamate-activated NMDA receptors are involved, and the relative importance of the NMDA and non-NMDA receptors as well as the involvement of other ion channels has been studied using a combination of electrophysiological recordings and modelling experiments, but little attention had been paid to their evolution during development. In the present experiments, which have been performed on Xenopus embryos from stages 31 to 42, we have probed the relative importance of the two categories of receptors using selective blockers [respectively dl-2-amino-5-phosphonovaleric acid (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)]. The sensitivity of the swimming behaviour to APV was found to increase during development and that to CNQX to decrease. Furthermore, it has been observed that the spike activity recorded from the ventral roots is more complex in late embryonic stages that in early embryos. These modifications are associated with changes of the neuronal circuit, some of which correspond to a lengthening of the axon and an increased complexity of the dendritic tree of the motoneurons. We have incorporated these modifications in a simplified model of the central pattern generator built with Neuron software. The results indicate that at least part of the observed changes can be associated with changes in the length of the dendrites and axons.