Recent publications have demonstrated an important role for olfactory afferent innervation in maintenance of the dopamine phenotype of olfactory bulb target neurons. The mechanisms underlying the control of phenotypic expression in this system are not known. These studies employed the model of unilateral neonatal olfactory deprivation to investigate the effects of lack of odorant stimulation on dopamine expression in the rat. Immunoreactivity of tyrosine hydroxylase, the first and rate-limiting enzyme in dopamine biosynthesis, used as a marker of the dopamine system, exhibited a large decrease both 40 and 70 days following olfactory deprivation. The losses were region specific suggesting that the deprivation was not complete. The number of immunoreactive GABAergic neurons was not reduced. The number of neurons containing aromatic L-amino acid decarboxylase (the second enzyme in the dopamine biosynthetic pathway) was also not decreased. Olfactory marker protein immunoreactivity in the glomeruli, a marker for afferent innervation, was not significantly altered indicating that the olfactory bulb was not denervated. These data demonstrate that neonatal deprivation, and the resulting lack of odorant stimulation, produces a transneuronal alteration in dopamine expression without neuronal loss. The studies also suggest that neuronal activity or the activity-dependent release of a trophic factor is necessary for the expression of the dopamine phenotype.