We have characterized the effects of laminin and a basement membrane extract (BME) on the morphology of embryonic rat sympathetic neurons maintained in tissue culture in the absence of nonneuronal cells. Neurons were grown on polylysine-coated coverslips in the presence or absence of laminin or BME in serum-free medium. Axons were distinguished from dendrites using intracellular dye injections, immunocytochemistry, and [3H]uridine autoradiography. In short-term (less than or equal to 24 hr) culture, laminin had a potent neurite-promoting effect, causing increases in the number of processes, total neuritic length, and neuritic branching. In long-term (3-35 days) cultures chronically exposed to laminin, most (greater than 75%) neurons maintained supernumerary axons but failed to form dendrites. In contrast, most neurons (greater than 70%) grown in long-term culture on polylysine in the absence of laminin were unipolar, extending a single axon. BME caused sympathetic neurons to extend multiple (range, 1-15) dendrites. Morphometric measurements made after 1 month of exposure to BME indicated that the amount of dendritic growth that occurred in vitro was similar to that normally occurring during a comparable period in situ. BME did not cause changes in the number of axons per neuron or in the uptake of neurotransmitter. Preliminary characterization of the dendrite-promoting activity of BME suggests that it resides in extracellular matrix (ECM) molecules and not in low-molecular weight contaminants. These observations indicate that (1) axonal and dendritic growth may be differentially regulated by various constituents of the ECM, and (2) such process-specific interactions can significantly affect the morphological development of sympathetic neurons.