Neurotrophin regulation of neuronal morphology is complex and may involve differential action of alternative Trk receptor isoforms. We transfected ferret visual cortical slices with full-length and truncated TrkB receptors to examine their roles in regulating cortical dendrite development. These TrkB isoforms had differential effects on dendritic arborization: whereas full-length TrkB increased proximal dendritic branching, truncated TrkB promoted net elongation of distal dendrites. The morphological effects of each receptor isoform were distinct, yet their actions inhibited one another. Actions of the truncated TrkB receptor did not involve unmasking of endogenous TrkC signaling. These results suggest that TrkB receptors do not regulate dendritic growth per se but, rather, the mode of such growth.