A variety of mechanisms have been proposed to explain the forward extension of cytoplasm in advancing cells and axonal growth cones, including actin polymerization and osmotic swelling. Based on our observations of the filopodia of cultured neuronal growth cones, we propose a mechanism involving motor-induced extension and retraction. We observed that filopodia (actin-based protrusions 0.2-0.5 mu in diameter) extend and retract from growth cone lamellae at the same rate. Further, force is generated at the tips of filopodia which is sufficient to produce compressive buckling of the proximal portion of the filopodium. From our analysis of these movements we suggest that a motor protein powers both the extension and retraction of filopodia.