Earlier studies in our laboratory have demonstrated that a discrete region of the anterior part of the neonatal subventricular zone (SVZa) contains exclusively neuronal progenitor cells. The descendants of the SVZa progenitor cells are destined for the granule cell and glomerular layers of the olfactory bulb, where they differentiate into granule and periglomerular cells, the interneurons of the olfactory bulb, respectively. In the present set of experiments we examined the neurotransmitter phenotype of the SVZa-derived cells. In order to label SVZa-derived cells, the cell proliferation marker bromodeoxyuridine (BrdU) was injected into the SVZa of postnatal day 2 (P2) rats. After 3 weeks, by which time most of the SVZa-derived cells have migrated to their final destination in the bulb, the animals were perfused and their brains processed for immunohistochemistry. To identify the neurotransmitter phenotype of the SVZa-derived cells, sagittal sections of the forebrain, including the olfactory bulb, were double-labeled with an antibody to BrdU in conjunction with an antibody to gamma-amino-butyric acid (GABA) or tyrosine hydroxylase (TH), the rate limiting enzyme in the synthesis of dopamine. Using simultaneous indirect immunofluorescence to detect the presence of single- and double-labeled cells, we found that 59% and 51% of the BrdU-positive cells were immunoreactive for GABA in the granule cell and glomerular layers, respectively. In addition, 10% of the BrdU-positive periglomerular cells were immunoreactive for TH. The presence of double-labeled (BrdU-positive/GABA-positive and BrdU-positive/TH-positive) cells in the olfactory bulb, demonstrates that the SVZa is a source of the GABAergic and dopaminergic interneurons of the olfactory bulb during postnatal development.