AbstractThe extension of filopodia from growth cones of regenerating neurites from rat superior cervical ganglion neurons in tissue culture was studied. Cultures were grown on a thin layer of fibrous collagen and maintained in a medium containing serum and nerve growth factor. Time‐lapse cinematography and computer‐assisted morphometry were used to observe and measure the kinetics of extension of individual filopodia. Filopodia extended from the growth cone margin, trailing neurite, or from each other. Frequently, extension was preceded by the appearance at the cone margin of a nodule of cytoplasm which appeared dense in phasecontrast optics. Branch points between adjacent extending filopodia remained fixed with respect to the growth cone while the filopodia lengthened. The rate of extension was maximum just after initiation (0.12 ± 0.4 μm/sec; mean ± SD; n = 36) and declined thereafter until the filopodium collapsed. This initial rate of extension was directly correlated with the eventual length of the filopodium (r = 0.67). Filopodia of growth cones arising from embryonic neurons exhibited higher initial extension rates (range: 0.07 to 0.20 μm/sec; mean = 0.13 μm/sec) than those of postnatal neurons (range: 0.01 to 0.13 μm/sec; mean = 0.09 μm/sec). These data are discussed in relation to a model proposed by Tilney and Inoue [1982] for the extension, by distal addition of G‐actin to growing filaments, of another type of elongating process filled with microfilaments, the acrosomal process o