Thymol is a natural essential oil present in many plants and has many different effects in various cell types. However, the effect of thymol on the physiology of MDCK renal tubular cells is unknown. The action of the phytochemical thymol on cytosolic Ca²⁺ concentrations ([Ca²⁺]i) and apoptosis in Madin-Darby canine kidney (MDCK) renal tubular cells was explored. Fura-2, a Ca²⁺-sensitive fluorescent dye, was used to assess [Ca²⁺]i. Thymol at concentrations of 200-500 μM caused a [Ca²⁺]i rise in a concentration-dependent manner. Removal of extracellular Ca²⁺ partially reduced the effects of thymol. Thymol-induced Ca²⁺ entry was inhibited by nifedipine, econazole, SK&F96365 and protein kinase C modulators. In a Ca²⁺-free medium, treatment with the endoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin inhibited thymol-induced [Ca²⁺]i increases. Treatment with thymol also inhibited thapsigargin-induced [Ca²⁺]i rise. Thymol killed cells at concentrations of 300-500 μM in a concentrationdependent fashion. Chelating cytosolic Ca²⁺ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/AM (BAPTA/AM) did not prevent thymol cytotoxicity. Thymol (400 and 500 μM) induced apoptosis detected by using Annexin V/propidium iodide staining. At 400 or 500 μM, thymol increased levels of reactive oxygen species. Together, in MDCK cells, thymol induced a [Ca²⁺]i rise by inducing Ca²⁺ release from the endoplasmic reticulum and Ca²⁺ entry via protein kinase C-sensitive store-operated Ca²⁺ channels. Our data suggest that thymol-induced apoptosis might involve reactive oxygen species (ROS) production.