The ventral part of the oral pontine reticular nucleus (vRPO) is involved in the generation and maintenance of rapid eye movement (REM) sleep. Both GABAergic and serotonergic neurotransmission have been implicated in the control of the sleep-wakefulness cycle. Nevertheless, the synaptic organization of serotonergic terminals in the vRPO has not yet been characterized. We performed an electron microscope study of serotonin-immunoreactive (5-HT-IR) terminals using immunoperoxidase or immunogold-silver methods. In a second set of experiments, combining GABA immunoperoxidase and 5-HT immunogold-silver techniques, we examined inputs from GABA-immunoreactive (GABA-IR) terminals to serotonergic neurons. 5-HT-IR terminals were located primarily on dendrites and occasionally on somata of unlabeled and 5-HT-IR neurons. The majority of the synapses formed by 5-HT-IR terminals were of the symmetrical type, making contacts primarily with unlabeled dendritic profiles. Moreover, 5-HT-IR terminals contacted unlabeled axon terminals that formed asymmetric synapses on dendrites. Double immunolabeling experiments showed 5-HT-IR and GABA-IR afferents, in apposition to each other, making synapses with the same dendrites. Finally, GABA-IR terminals innervated 5-HT-IR and GABA-IR dendrites. Our findings indicate that serotonin would modulate the neuronal activity through inhibitory or excitatory influences, although the action of serotonin on the vRPO would predominantly be inhibitory. Moreover, the present results suggest that the serotonin modulation of vRPO neurons might involve indirect connections. In addition, GABA might contribute to the induction and maintenance of REM sleep by inhibiting serotonergic and GABAergic neurons in the vRPO.