A comparative study of cholera toxin (CT) and wheat germ agglutinin (WGA) conjugated with horseradish peroxidase (HRP) was made on trigeminal central projections of the lower incisor gingiva afferent neurons in the guinea pig. Considerably more CT-HRP-labeled endings were observed in the trigeminal sensory nuclear complex (TSNC) and in the cervical spinal cord (C1-C8). The substantia gelatinosa (lamina II) of both the caudal nucleus of the TSNC and C1-C2 was the only area where WGA-HRP labeled more terminals. CT-HRP-labeled fibers and endings were traced up to C7-C8, whereas with WGA-HRP were rare caudal to C5. A comparison of the two methods currently in use, i.e. the 2-step glutaraldehyde and sodium periodate, showed that the latter yields conjugates which are more sensitive as neuroanatomical tracers.