To gain insight into how laminin promotes neurite growth, high resolution video microscopy was used to determine the rapid effects of laminin on growth cone structure. Sympathetic growth cones in serum-free medium on polylysine substrate displayed extensive motility and protrusive activity and often had large lamellipodia. However, their neurites grew slowly because membranous organelles from the central region advanced into the lamellipodium only slowly. Acute addition of laminin accelerated growth severalfold and had visible effects on the growth cone within minutes. Laminin dramatically accelerated the advance of membranous organelles, which, with microtubules, rapidly filled the lamellipodium. Retraction of individual protrusions (filopodia and veils) was rapidly reduced. These effects of laminin are important in accelerating growth and suggest a mechanism for pathway selection by growing neurites.