The aim of these experiments was to analyze how depolarization influences neurite outgrowth in leech neurons and what role the substrate and Ca2+ play in this response. Neurons in culture were exposed to 60 mM extracellular K+ for 30 min, which induced retraction of a subset of neurites growing on extracellular matrix substrate (ECM), a response comparable to that observed after electrical stimulation (Grumbacher-Reinert and Nicholls, 1992). After normal medium had been restored, the neurites continued to retract for about 1 hr to approximately 80% of the total starting neurite length. Retraction was reversible and regrowth began after the cells had been in normal medium for about 3 hr. Similar depolarization-induced neurite retraction was observed in both Retzius and anterior pagoda cells. Retraction was inhibited by raised extracellular Mg2+, suggesting a mechanism dependent on calcium. The effect of high K+ on neurite outgrowth was also influenced by the substrate on which the cells were plated. Cells plated on concanavalin A (ConA) did not retract but continued to extend processes during exposure to high K+. To understand the different behavior of cells grown on ECM and ConA, the morphology of growth cones was analyzed by scanning electron microscopy. The growth cones of cells grown on ECM and exposed to high K+ revealed retraction of lamellipodial and filopodial structures. On ConA, however, no differences were observed between growth cones of cells exposed to high K+ and those of control cells. These results demonstrate the importance of substrate molecules in the responses of growth cones to depolarization and therefore in the differentiation of neurons.