When cultured on polylysine, rat sympathetic neurons extend modest lamellae which contain a mass of relatively short non-aligned microtubules. Microtubules display movements, but these movements do not result in any obvious alterations in the overall configuration of the array. Application of a mixture of growth factors called matrigel results in a rapid expansion of the lamellae followed by the outgrowth of axons. Microtubules undergo rapid behavioral changes that result in dramatic alterations in the microtubule array. Microtubules become significantly longer, and extend to the periphery of the lamellae where they invade newly-forming axons. The microtubules align with one another and relative to the cell cortex, and draw together into bundles. Microtubules within a bundle move apart as well, particularly at the tips of developing axons. These observations demonstrate a complexity of microtubule behaviors, some of which can be explained by interactions with actin and/or by forces generated by molecular motor proteins.