The mechanisms of H(2)O(2)-induced elevated calcium baselines in PC12 cells were investigated in the present study by using fura-2-fluorescent image analysis. The results showed that the calcium comes from both intracellular and extracellular sources. Although the major intracellular source was mitochondria, only the extracellular calcium influx was responsible for the sustained post-H(2)O(2)-exposure increases. This calcium influx was partially blocked by calcium channel antagonists [verapamil (L-type) or mibefradil (nonselective)] and was more effectively blocked by the sodium channel antagonist, tetrodotoxin (TTX). Membrane depolarization following H(2)O(2) exposure contributed to the opening of the ion channels. The H(2)O(2)-induced calcium influx was blocked by TTX even in a sodium-free buffer, indicating that calcium directly fluxed through sodium channels. Sodium-calcium exchangers (NCX) on the plasma membrane did not play a role, because use of a specific reverse mode NCX inhibitor, No. 7943, was ineffective in blocking the influx. The H(2)O(2)-induced calcium influx was mimicked by using a thiol-selective oxidizing reagent, 2', 2'-dithiodipyridine, and in both situations, the calcium levels were completely reversed by a thiol-selective reducing reagent, dithiothreitol. Our results indicated that mechanisms of oxidant-induced elevated calcium baselines in PC12 cells involved calcium influx through sodium and calcium channels that may be directly or indirectly attributed to thiol oxidation.