The influence of the extracellular potassium concentration ([K+]o) on neurite growth in rat sensory neurons, spinal cord neurons and sympathetic neurons was investigated. Experiments carried out in 3-compartment culture dishes showed that although neurites from sensory and spinal cord neurons were capable of growing in both 5 mM [K+]o and 20 mM [K+]o, they were virtually unable to grow from a region of 5 mM [K+]o into a region of 20 mM [K+]o. Neurites from sympathetic neurons behaved similarly although [K+]o exceeding 20 mM was required to exclude sympathetic neurites. We suggest the possibility of a negative chemotaxis to [K+]o by growth cones in these neurons. Neurite regeneration following axotomy in sensory neurons was partially inhibited distal to a proximo-distal increase in [K+]o. The nature of this inhibition was somewhat different from that described previously in sympathetic neurons. The possibility is raised that [K+]o plays a role in the development of the nervous system.