The motor program for swimming in the lamprey includes rhythmic bursting of motoneurons with output from the two sides of a spinal segment alternating strictly out of phase. This motor program can be observed in vitro in the isolated spinal cord. Addition of strychnine (5-10 microM) can selectively eliminate the alternation between sides without blocking the temporal pattern of bursting on each side. This result suggests that temporal bursting on the right and left sides of the spinal cord is generated by independent neuronal oscillators, while the alternation between the two sides results from crossed inhibitory coupling between these oscillators.