The distribution of dopamine immunoreactivity (DA-IR) in lamprey spinal cord was examined using antibodies to dopamine. This revealed a population of small cells slightly ventral to the central canal, with processes extending to it. Ventral to the cells is a sparse plexus of DA-IR fibers. Bath application of DA during glutamate-induced fictive swimming caused cycle rate acceleration at 0.1-1 microM, but slowed the rhythm at higher concentrations (10 microM-1 mM). Bath application or pressure ejection of DA onto spinal motoneurons reduced the late afterhyperpolarization phase of action potentials by 40-50%, without affecting input resistance, membrane potential, or action potential amplitude or duration.