Learning and memory depend upon neuronal alterations induced by electrical activity. Most examples of activity-dependent plasticity, as well as adaptive responses to neuronal injury, have been linked explicitly or implicitly to induction by Ca²⁺ signals produced by depolarization. Indeed, transient Ca²⁺ signals are commonly assumed to be the only effective transducers of depolarization into adaptive neuronal responses. Nevertheless, Ca²⁺-independent, depolarization-induced signals might also trigger plastic changes. Establishing the existence of such signals is a challenge because procedures that eliminate Ca²⁺ transients also impair neuronal viability and tolerance to cellular stress. We have taken advantage of nociceptive sensory neurons in the marine snail, Aplysia, which exhibit unusual tolerance to extreme reduction of extracellular and intracellular free Ca²⁺ levels. The axons of these neurons exhibit a depolarization-induced, memory-like hyperexcitability that lasts a day or longer and depends upon local protein synthesis for induction. Here we show that transient, localized depolarization of these axons in an excised nerve-ganglion preparation or in dissociated cell culture can induce short- and intermediate-term axonal hyperexcitability as well as long-term, protein synthesis-dependent hyperexcitability under conditions in which Ca²⁺ entry is prevented (by bathing in nominally Ca2+-free solutions containing EGTA) and detectable Ca²⁺ transients are eliminated (by adding BAPTA-AM). Disruption of Ca²⁺ release from intracellular stores by pretreatment with thapsigargin also failed to affect induction of axonal hyperexcitability. These findings suggest that unrecognized Ca²⁺-independent signals exist that can transduce intense depolarization into adaptive cellular responses during neuronal injury, prolonged high-frequency activity, or other sustained depolarizing events.