A spatially discrete region of the anterior part of the postnatal telencephalic subventricular zone, referred to as the SVZa generates vast numbers of lineally-related neurons destined for the olfactory bulb (Luskin, 1993). The cells originating in the SVZa migrate to the olfactory bulb along a highly restricted pathway which is in a direction orthogonal to the orientation of radial glial fibers. In this study we analysed the number, distribution, orientation and rate of migration of SVZa-derived cells as they approach the olfactory bulb. In order to track the SVZa-derived cells, a retroviral lineage tracer, encoding the reporter gene E. coli beta-galactosidase (lacZ) was injected precisely into the rat SVZa at postnatal day 1 (P1). The lacZ-positive cells were visualized 1, 2 and 3 days later by X-Gal histochemistry in cryostat sections. As the number of SVZa-derived cells in the pathway increased with survival time, their distribution changed systematically. The distribution pattern of lacZ-positive cells by 2 and 3 days postinjection suggested that some of the progeny of infected progenitor cells were undergoing neurogenesis as they proceeded to the olfactory bulb; a large percentage of the lacZ-positive cells were substantially displaced from the SVZa injection site. To investigate whether lacZ-positive cells migrate in a directed fashion, their orientation preference was scored. For the majority of lacZ-positive cells (> 94%), their leading process was directed toward the olfactory bulb, possibly reflecting a response to migratory cues present along the pathway. The estimated average rate of cell migration to the olfactory bulb was 23 mu m/h, which is approximately twice the speed of radially directed neuronal migration from the telencephalic ventricular zone to the cortical plate (O'Rourke et al., 1992). Collectively, these results suggest that SVZa-derived interneurons en route to the olfactory bulb may employ a novel mode of tangential migration.