The lipoxygenase (LO) inhibitors nordihydroguaiaretic acid (NDGA) and 16S-hydroxy-5,8,11,13-(Z,Z,Z,E)-eicosatetraenoic acid (15-HETE) have been found to suppress the rise in free cytoplasmic Ca2+concentration ([Ca2+]i) induced by the Ca2+ionophores ionomycin and A23187 in rat thymocytes. Bromophenacyl bromide (BPB), a phospholipase A2(PLA2) inhibitor, produced a much weaker inhibitory effect, and indomethacin, a cyclo-oxygenase inhibitor, practically did not influence the [Ca2+]iresponse to ionomycin. These findings implicate the involvement of LO product(s) in the [Ca2+]irise triggered by the Ca2+ionophores. The contribution of the NDGA-sensitive component to the ionomycin-induced [Ca2+]irise was significant in the ionomycin concentration range of 0.1 nM to 0.1 μM, whereas at higher doses of the ionophore it gradually diminished. By contrast, the [Ca2+]irise induced by exogenous arachidonic acid (AA) or melittin, a PLA2activator, was not suppressed but potentiated by NDGA. Ionomycin and exogenous AA also elicited opposite changes in thymocyte cytoplasmic pH (pHi): the former elevated the pHiwhile the latter induced a pronounced acidification of the cytoplasm. This difference in the pHiresponses may account for the different sensitivity of ionomycin- and AA-elicited [Ca2+]isignal to LO inhibitors. © 1990.