The synaptic organization of the motor trigeminal nucleus in adult rats treated neonatally with 6-hydroxydopamine (6-OHDA) was investigated quantitatively and compared with control nuclei. No statistically significant change was detected in the distribution of axon terminals in the neuropil, and the total number of axosomatic contacts per unit length of membrane was identical in the control and 6-OHDA-treated groups. However, 6-OHDA treatment causes a significant redistribution of the four morphologically distinct bouton populations forming axosomatic contacts with trigeminal motoneurons. Terminals containing lucent axoplasm and spherical synaptic vesicles have been identified as norepinephrine neuron terminals (Card et al.: J. Comp. Neurol. 250:469-484, '86). These and terminals with lucent axoplasm and pleomorphic vesicles are increased in number whereas terminals with dense axoplasm and either spherical or pleomorphic synaptic vesicles are decreased in number in the 6-OHDA-treated brains compared to controls. These results confirm that the norepinephrine hyperinnervation observed in histofluorescence preparations following neonatal 6-OHDA treatment reflects an increase in absolute numbers of norepinephrine terminals. The finding that the total number of axosomatic contacts per unit length of membrane remains constant while the proportions of individual afferent classes vary may indicate that the trigeminal motoneuron plays a major role in determining the overall density but not necessarily the individual specificity of its axosomatic innervation. The motor trigeminal nucleus is a useful model system in which to investigate both the response of norepinephrine fibers to neonatal 6-OHDA treatment and the respective roles of a target neuron and its afferents in the regulation of appropriate quantitative innervation patterns in the central nervous system.