Transient increases in extracellular K + are observed under various conditions, including repetitive neuronal firing, anoxia, ischemia and hypoglycemic coma. We studied changes in cytoplasmic Ca 2+ ([Ca 2+] cyt) evoked by pulses of KCl in human neuroblastoma SH-SY5Y cells and rat dorsal root ganglia (DRG) neurons at 37 °C. A “pulse” of KCl evoked two transient increases in [Ca 2+] cyt, one upon addition of KCl (K + on) and the other upon removal of KCl (K + off). The K + on transient has been described in many cell types and is initiated by the activation of voltage-dependent Ca 2+ channels followed by Ca 2+-evoked Ca 2+ release from intracellular Ca 2+ stores. The level of KCl necessary to evoke the K + off transient depends on the type of neuron, in SH-SY5Y cells it required 100 mM KCl, in most (but not all) of dorsal root ganglia neurons it could be detected with 100–200 mM KCl and in a very few dorsal root ganglia neurons it was detectable at 20–50 mM KCl. In SH-SY5Y cells, reduction of extracellular Ca 2+ inhibited the K + on more strongly than the K + off and slowed the decay of K + off. Isoflurane (1 mM) reduced the K + on- but not the K + off-peak. However, isoflurane slowed the decay of K + off. The nonspecific cationic channel blocker La 3+ (100 μM) had an effect similar to that of isoflurane. Treatment with thapsigargin (TG) at a concentration known to only deplete IP3-sensitive Ca 2+ stores did not affect K + on or K + off, suggesting that Ca 2+ release from the IP3-sensitive Ca 2+ stores does not contribute to K + on and K + off transients and that the thapsigargin-sensitive Ca 2+ ATPases do not contribute significantly to the rise or decay rates of these transients. These findings indicate that a pulse of extracellular K + produces two distinct transient increases in [Ca 2+] cyt.