Transneuronal anterograde labeling with the conjugate wheat germ agglutinin-horseradish peroxidase (WGA-HRP) has been documented in the mammalian and immature avian visual system [6,14]. Transneuronal retrograde labeling was significant only in the chick [6]. The present study was performed to determine whether transneuronal labeling could be shown in the mammalian olfactory system, whether the phenomenon was robust in adults, and whether transneuronal retrograde transport could label several transmitter-specific centrifugal afferent projections to the olfactory bulb. In addition we wished to learn whether molecules that enter the nasal cavity can undergo transport to brain neurons. Gelfoam implants soaked with 1% WGA-HRP, surgically implanted into the nasal cavity, produced transneuronal labeling patterns that affirmed all of these questions. Transneuronal anterograde transport labeled the appropriate zones in the olfactory bulb and in all second order olfactory targets. In addition, there was transneuronal retrograde labeling of neurons in the olfactory bulb, anterior olfactory nucleus and in transmitter-specific projection neurons from the diagonal band (cholinergic), raphe (serotonergic) and locus coeruleus (noradrenergic). Transneuronal labeling was robust and consistent. The patterns of labeling indicated that transneuronal anterograde and retrograde transport occurred along known, specific circuits in the olfactory system. The present results suggest that nasal epithelial application of WGA-HRP may be a useful tool for assessing regeneration of primary olfactory neurons and the status of central circuitry following regeneration. The method should also facilitate the study of central olfactory connections after surgical or genetic lesions of the olfactory bulb. Finally, these experiments suggest the possibility that inhaled molecules including, possibly substances of abuse, may be transported to, and, possibly, influence the function of neurons in the brain, including some (diagonal band, raphe, locus coeruleus) which have extensive projections to wide areas of the CNS.