The importance of cytosolic free calcium level ([Ca2+]i) in lymphocyte activation prompted us to investigate changes in [Ca2+]i in T cells caused by mercury compounds, which have been shown to have immunomodulatory and immunotoxic properties. Using fura‐2 as fluorescent Ca2+indicator, we found that both methyl‐mercury (MeHg; 0.02–2 μM) and inorganic mercury (HgCI2; 0.01–1 μM) increased [Ca2+]i in lymphocytes from rat spleen in a concentration‐dependent manner. The effect of MeHg was rapid and the increase of Ca2+level was sustained in time, while HgCI2caused a slow rise in [Ca2+]i. The effects of mercury compounds did not appear to be associated with alterations of membrane integrity, since there was no significant difference in the extent of MnCI2quench between control and mercury‐treated cells. However, HgCI2(1 μM) and MeHg (2 μM) appeared to cause membrane damage at longer incubation times (15 min). When cells were incubated in Ca2+‐free medium (in the presence of 1 mM EDTA) MeHg still increased [Ca2+]i, though to a lesser extent, while HgCI2had no effect. Heparin, an inhibitor of inositol 1,4,5,‐trisphosphate‐induced Ca2+mobilization partially blocked this rise of [Ca2+]i, while carbonyl cyanide m‐chlorophenylhydraxone (CCCP), an inhibitor of mitochondrial function, had a lesser effect. When added together, heparin and CCCP almost completely block the response to MeHg. These results suggest that MeHg and HgCI2exert their effects of [Ca2+]i in different ways: MeHg‐induced increases in [Ca2+]i are due to influx from outside the cells as well as to mobilization from intracellular stores, possibly the endoplasmic reticulum, and, to a minor extent, the mitochondria; on the other hand, HgCI2causes only Ca2+influx from the extracellular medium.