In larvae of the amphibian, Xenopus laevis, spinal neurons which are active during fictive swimming also display tetrodotoxin-resistant membrane potential oscillations following the coactivation of N-methyl-DL-aspartate (NMDA) and 5-hydroxytryptamine (serotonin or 5-HT) receptors (Scrymgeour-Wedderburn et al., 1997; Eur. J. Neurosci., 9, 1473-1482). The oscillations are slow (approximately 0.5 Hz) compared with swimming (approximately 7-35 Hz) raising doubt over their contribution to the cycle by cycle depolarizations occurring during swimming. We investigated an alternative: that the intrinsic oscillations modulate swimming activity over many consecutive cycles. Bath application of NMDA induced continuous fictive swimming that differed between embryonic and larval preparations. In 81% of larval preparations (n = 36), there was a slow (approximately every 2 s) rhythmic modulation of ventral root activity in which burst durations and intensities increased as cycle periods decreased. This pattern of activity was enhanced rather than abolished following blockade of glycine and gamma-aminobutyric acid (GABA) A receptors and presumably therefore resulted from a periodic increase in the excitation of motor neurons. To determine whether this slow rhythm resulted from intrinsic, 5-HT-dependent membrane potential oscillations, larvae were spinalized to prevent the release of 5-HT from brainstem raphe neurons. The resulting pattern of NMDA-induced activity lacked any slow modulation. The slow modulation could also be enhanced by the bath application of a 5-HT receptor agonist (5-carboxamidotryptamine) and abolished either by the addition of an antagonist (pindobind-5-HT1A) or by removal of magnesium ions, providing more direct evidence for a contribution of intrinsic oscillations. Thus, the 5-HT-dependent intrinsic oscillations modulate NMDA-induced swimming activity over several consecutive cycles.