Rhodamine 123 is a fluorescent dye that localizes in mitochondria, is a substrate for the multidrug resistance pump, and is retained for long periods of time by carcinoma cells. 17 beta-Estradiol causes GH4C1 cells (rat pituitary tumor cells) to lose rhodamine 123 fluorescence faster than untreated cells. We found that estradiol induces accumulation of the mRNA for the multidrug resistance pump 3-5-fold, with maximum induction occurring within 1 day at 10(-9) M estradiol. Immunoblot analysis demonstrated that estradiol induces a protein of 150 kDa that reacts with an antibody to P-glycoprotein, the multidrug resistance pump. The reduced retention of rhodamine 123 caused by estradiol is prevented by verapamil and cyclosporin, inhibitors of the pump. A clone resistant to the effects of estradiol on rhodamine 123 has greatly reduced levels of mRNA for the pump. The effect of estradiol is more marked on rhodamine 123 retention than it is on that of rhodamine 110 or tetramethylrhodamine methyl ester. We conclude that estradiol enhances rhodamine 123 efflux by inducing the multidrug resistance gene. The specificity for rhodamine 123, compared with other analogs, may be caused by differences in accessibility to the pump.