THE addition of plasma membrane proteins to a growing axon could occur by preferential insertion at the tip (the growth cone), by uniform insertion along the axon, or by insertion at the cell body and bulk flow along the axon. To differentiate between these possibilities we used a defective herpesvirus vector to express an exogenous protein, the lymphocyte transmembrane protein CD8 alpha, in cultured rat hippocampal neurons. The newly synthesized protein first appeared on the axonal surface almost exclusively at the growth cone. Preferential addition at the growth cone was also observed in minor processes (immature dendrites), but not in mature dendrites. Over several hours, CD8 alpha reached a uniform distribution over the entire neuronal surface, presumably by diffusion within the membrane and possibly endocytic recycling. As well as providing materials for axonal growth, the selective addition of membrane vesicles at the growth cone may contribute to the polarized distribution of axonal surface molecules(1).