NEURONS in the central nervous system (CNS) often store more than one neurotransmitter(1,2), but as yet the functional significance of this type of coexistence is poorly understood. 5-Hydroxytryptamine (5-HT) modulates calcium-dependent K+ channels (K-Ca) responsible for the postspike afterhyperpolarization in different regions of the CNS3,4. In lamprey, 5-HT neurons control apamine-sensitive K-Ca channels in spinal locomotor network interneurons(4-6), thereby in addition regulating the duration of locomotor bursts(7,8). We report here that these spinal 5-HT neurons also contain dopamine. Like 5-HT, dopamine causes a reduction of the afterhyperpolarization, but in this case it is due to a reduction of calcium entry during the action potential, which results in a reduced activation of Kc,. 5-HT and dopamine are both released from these midline neurons, and both reduce the afterhyperpolarization through two distinctly different, but complementary cellular mechanisms. The net effect of dopamine (10-100 mu M) on the locomotor network is similar to that of 5-HT, and the effects of dopamine and 5-HT are additive at the network level.