The role of apoptosis and the implications of the functions of apical membranes in cisplatin-induced nephrotoxicity were investigated using the kidney epithelial cell line LLC-PK₁. When LLC-PK₁ cells were treated with 30 μM cisplatin, the number of floating cells was increased markedly. However, the number was not increased by treatment with 1 mM cisplatin, suggesting that different mechanisms were involved in the toxicities of these two treatments. DNA fragmentation, condensation of nuclear chromatin, and the absence of trypan blue staining suggested that cellular toxicity following treatment with 30 μM cisplatin for 24 hr was mediated predominantly by apoptosis. Specific activities of apical enzymes (γ-glutamyltransferase, EC 2.3.2.2; and alkaline phosphatase, EC 3.1.3.1) in LLC-PK₁ cells were decreased markedly by treatment with 30 μM cisplatin for 24 hr, whereas neither lactate dehydrogenase (LDH; EC 1.1.1.27) release nor a decrease in cellular protein content was observed following the same treatment. In addition, concomitant treatment with reduced glutathione completely attenuated both the apoptosis and the decrease of apical enzyme activities induced by 30 μM cisplatin. Neither DNA fragmentation nor condensation of chromatin was induced by treatment with 1 mM cisplatin for 12 hr. However, LDH release and a decrease in cellular protein level were induced by 1 mM cisplatin, suggesting that the toxic effect was due to necrosis. Under these conditions, specific activities of apical enzymes were not decreased. These results suggested that apoptosis was more responsible than necrosis for the loss of apical functions in cisplatin-induced toxicity in LLC-PK₁ cells.