Peripheral deafferentation of the rodent olfactory bulb results in loss of dopamine content, tyrosine hydroxylase activity and immunocytochemical staining for tyrosine hydroxylase in juxtaglomerular dopamine neurons. Reinnervation of the bulb by afferent neurons results in the return of all parameters to control levels suggesting that the dopamine neurons did not degenerate but that the expression of tyrosine hydroxylase enzyme was transneuronally regulated in a static population of juxtaglomerular cells. To evaluate this possibility, we determined the activity and immunocytochemical localization of the second enzyme in the dopamine biosynthetic pathway, DOPA decarboxylase. At a time when tyrosine hydroxylase activity was reduced to 25% of control values, DOPA decarboxylase activity in the lesioned bulb was maintained at about 65% of that in the unlesioned bulb. Immunocytochemical staining with antibodies to both enzymes, performed sequentially in the same sections, demonstrated that in the unlesioned bulb tyrosine hydroxylase and DOPA decarboxylase are co-localized in the same population of juxtaglomerular neurons. Similar results were obtained in adjacent sections each stained with one of the two antibodies. In contrast, in the deafferented bulb, about three times as many neurons were stained with DOPA decarboxylase as with tyrosine hydroxylase antibodies. The DOPA decarboxylase activity measurements and immunocytochemistry argue for the continued presence, in the lesioned olfactory bulb, of a population of tyrosine hydroxylase deficient dopamine neurons. The data suggest that olfactory receptor cell innervation transneuronally regulates the expression of tyrosine hydroxylase by mechanisms separate from those controlling the levels of DOPA decarboxylase.