Etoposide, used for the treatment of breast cancer, is mainly metabolized via hepatic cytochrome P450 (CYP) 3A4 in humans and is also a substrate for p-glycoprotein (P-gp). Morin is known to be able to modulate the activities of metabolic enzymes including CYPs and can act as a potent P-gp inhibitor. The purpose of this study was to investigate the effects of morin on the pharmacokinetics of etoposide in rats with 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumors. Etoposide was administered intravenously (2 mg/kg) and orally (10 mg/kg) in control and DMBA rats without (DMBA-WOM) and with (DMBA-WM) morin (15 mg/kg). Protein and mRNA expression of CYP3A and P-gp was analyzed, and the tissue distribution of etoposide was also measured. Both protein and mRNA expression of CYP3A and P-gp was inhibited by morin in the liver, intestine and breast tumors of DMBA-WM rats. After both intravenous and oral administration of etoposide in DMBA-WM rats, the total area under the plasma concentration-time curve from time zero to infinity (AUC) of etoposide was significantly greater, and the time-averaged total body clearance (CL) of etoposide was significantly slower than those in control and DMBA-WOM rats. The amount of etoposide recovered from each tissue was significantly higher in DMBA-WM rats, especially in the breast tumor, liver and large intestine. No significant differences between control and DMBA-WOM rats were observed. Taken together, greater AUC and slower CL of etoposide in DMBA-WM rats could possibly be due to the inhibition of hepatic CYP3A (intravenous) and mainly due to the inhibition of intestinal CYP3A and P-gp (oral) by morin.