The truncated isoform of TrkB, TrkB.T1, has been shown to be expressed in the neurogenic region of rodent brain. TrkB.T1 lacks tyrosine kinase activity and it may modify the action of the full-length TrkB. We show here that the full-length TrkB and TrkB.T1 are expressed at the same relative expression levels in mouse neural progenitor cell cultures. The number of neurosphere-forming progenitors was reduced and apoptosis increased in neurospheres generated from mice overexpressing TrkB.T1 when compared with wild-type neurospheres. The proliferation of the transgenic neural progenitors was increased, as indicated by the larger average diameter of spheres (140% of control), the increased cell growth in an MTT assay (137% of control) and the faster rate of 3H-thymidine incorporation (128% of control) in the transgenic cell cultures than in controls. The proliferation of neural progenitors was also increased after lentivirus-mediated TrkB.T1 overexpression. A significant increase in 3H-thymidine incorporation (119% of control) and the average diameter of spheres (112% of control) in the TrkB.T1-transduced neurospheres compared with neurospheres transduced with the control vectors confirmed the role of TrkB.T1 in proliferation of neural progenitor. When induced to differentiate, progenitors overexpressing TrkB.T1 generated two to three times more neurons than did wild-type cells. The increase in the number of neurons correlated with an increase in the number of apoptotic cells (two-fold) at these time points. The data indicate that changes in the relative expression levels of different TrkB isoforms influence the replicative capacity of neural progenitors.