In the first 24 h of post-embryonic development, the motor rhythm underlying swimming in Xenopus laevis tadpoles changes from brief (ca. 7 ms) ventral root discharge in each cycle to bursts of activity lasting around 20 ms (Sillar et al. 1991). Because individual motoneurons in the spinal cord of newly hatched embryos normally fire only a single impulse per cycle, two possible changes underly the transition to motor bursts seen in larval ventral roots; desynchronization of neurons in a given ventral root which continue to fire once per cycle, or the developmental acquisition of a multiple spike capability in individual motoneurons. Here we have recorded intracellularly from ventrally positioned spinal neurons, presumed to be myotomal motoneurons, in stage 37/38 embryos and 24 h later in development in stage 42 larvae. We find that (i) larval neurons are able to fire more than one impulse per cycle of fictive swimming activity; (ii) unlike in the embryo, they generally will fire multiple impulses in response to injected depolarizing current; (iii) the synaptic drive to motoneurons during swimming increases dramatically in complexity, although it still consists of alternating phases of synaptic excitation and chloride-dependent inhibition, superimposed upon tonic synaptic depolarization. The results therefore suggest a developmental change in the membrane properties of rhythmically active neurons as a major factor in the post-embryonic development of swimming in Xenopus larvae. This change appears to occur in premotor rhythm generating interneurons as well as in the motoneurons themselves and may satisfy a demand for behavioural flexibility that allows larvae to survive in a complex and changing environment.