The neuroanatomy of a serotonin neurohemal system in the head of Periplaneta americana was studied by means of immunohistochemistry, cobalt backfilling, transmission electron microscopy, and nerve transection. This neurohemal system is supplied by bilateral groups of two or three neurons whose somata are located ventrally in the subesophageal ganglion, near the root of each mandibular nerve. Axons of these serotoninergic neurons extend into all of the nerves of the mouth parts but reach most of these nerves by a very circuitous route. Initially the axons extend from the subesophageal ganglion, through the ipsilateral mandibular nerve trunk, and into the third branch of the mandibular nerve. From here the axons extend into the second branch of the maxillary nerve by way of a link nerve, and then they project retrogradely to reenter the subesophageal ganglion. In the ganglion, branches of these axons extend into the labial nerves, and the axons run dorsally through the subesophageal ganglion, circumesophgeal connectives, and tritocerebrum to reach the labral nerves. In the nerves of the mouth parts the serotoninergic axons give rise to numerous secondary branches that form an extensive neurohemal system at the surface of these nerves. The relatively large surface and cephalic location of this system probably indicate that the timely release of relatively large amounts of serotonin plays an important role in the physiology of feeding in this insect. The somata, neurites, and dendritic fields of the serotonin neurohemal neurons and those of the motor neurons of the mandibular abductor muscle occur together, and some of the mandibular abductor motor neurons also stain for serotonin. In order to distinguish clearly between these neurohemal and motor neurons, the anatomy of the mandibular abductor motor neurons has also been determined. Similarly, in the course of this study it has been necessary to work out the anatomy of the motor neurons of the maxillary retractor and cardo rotator muscles in order to distinguish them from the serotoninergic neurons. A nonserotoninergic peripheral neuron is associated with the serotonin neurohemal system, and its soma is located on the mandibular-maxillary link nerve. This link nerve neuron appears to be neurosecretory.