Megestrol acetate (MGA) is being widely used for the improvement of appetite and performance status in patients receiving chemotherapy, especially cisplatin-containing therapy. However, little is known about whether MGA has an effect on cisplatin cytotoxicity. We have investigated this using two transitional carcinoma cell lines, i.e. the cisplatinsensitive parental line NTUB1 and the resistant daughter line NTUB1/P. Combined effects of MGA and cisplatin were assayed with a microculture chemosensitivity method. We explored the level changes of several cisplatin detoxification mechanisms, including metallothionein (MT), glutathione Stransferase- π (GST-π ) and glutathione (GSH) levels in cells treated with or without MGA. After treatment with 10µm MGA for 24 h, the cisplatin IC50s of NTUB1 and NTUB1/P increased 1.4- (p=0.03) and 1.6- (p=0.02) fold, respectively. By median effect analysis, the combinations of MGA and cisplatin in the two cells appeared to produce an antagonistic interaction. By Northern analysis, MT transcript levels in both cells were significantly upregulated after treatment with MGA, as compared to those without treatment. Exposure to MGA in either sensitive or resistant cells did not alter GST-n levels as shown by immunoblotting analysis. Cellular GSH content was increased only in NTUB1/P (p=0.0036) but remained unchanged in NTUB1 cells (p=0.29) after MGA exposure. In conclusion, MGA may antagonize cisplatin cytotoxicity by upregulating cellular MT and GSH levels. Use of MGA in cisplatin-containing chemotherapy may impair tumor response by antagonizing cisplatin antitumor activity.