The basis for longitudinal coordination among spinal neurons during locomotion is still poorly understood. We have now examined the functional projection distances for the longitudinal axons of reciprocal inhibitory 'commissural interneurons' in the spinal cord of young Xenopus tadpoles. In quiescent animals, glycinergic inhibitory postsynaptic potentials (IPSPs) were evoked in ventral spinal neurons by stimulating small rostral and caudal groups of commissural interneuron somata at different distances on the opposite side of the hindbrain and spinal cord. Unitary IPSPs, produced by single synaptic contacts, could be distinguished from background noise. Local cord stimulation at different distances revealed maximum functional projection distances up to approximately 0.5 mm for both descending and ascending axons, but with the probability of recording connections falling steeply over this distance. These maximum longitudinal projection distances are smaller than predicted by axonal anatomy (approximately 1.2 mm). We then measured functional projection distances during swimming by examining the synaptic output of a surgically isolated group of rostral commissural interneurons, mapping the occurrence of the mid-cycle, reciprocal IPSPs they produced in more caudal neurons. IPSPs occurred with high probability up to 0.9 mm away, nearly twice the projection distance found in quiescent tadpoles. These results show that synaptic contacts from commissural interneurons could influence longitudinal coupling during swimming at distances of up to 0.9 mm (approximately 4-5 myotome segments or approximately 25% of the spinal cord). They provide direct evidence for functional projection distances of a characterized class of interneurons belonging to a spinal locomotor pattern generator.