The morphological characteristics and distribution of serotonin-immunoreactive terminals within the rat mesencephalic trigeminal nucleus (Me5) were studied using immunocytochemical electron microscopy. Fibers, immunostained by specific antibodies raised against serotonin, were distributed throughout the entire rostrocaudal portion of the Me5. Examination of 355 serotonergic terminals in the caudal part of the Me5 revealed that 93% formed synaptic contacts with dendritic shafts: 68% on small (< 1.5 micron diameter) and 25% on large (> 1.5 micron diameter) dendrites. The remaining 7% formed axosomatic synapses, about a third of which are in contact with small spinous processes (somatic spines). Serotonin was also present in vesicle-filled boutons lacking synaptic-membrane specializations. Our data indicate that of all the dendritic synaptic and nonsynaptic inputs observed in the Me5 nucleus, the serotonergic component amounted to between 15 and 27%. More than half (58%) of the somatic synaptic input on the Me5 cells appears to be serotonergic. This suggests that, of all the somatic synaptic input on the proprioceptive primary sensory neurons of the Me5, the serotonergic afferent input probably has the most significance for the functioning of Me5 neurons.