Irinotecan (CPT-11) is a broad spectrum agent for the treatment of solid tumor malignancies, despite severe diarrhea is limiting its widespread usage. The local effects of SN-38 in the small intestine were considered to be responsible for the irinotecan-induced delayed diarrhea. It was proposed that cyclosporin A (CsA) inhibiting biliary excretion could attenuate this side effect, but in fact, it could not improve the therapeutic index of irinotecan. At present, most studies focused on the inhibition of bile excretion by cyclosporin A through the transporters MRP2 and MDR1 and its effect the irinotecan treatment in vivo. However, UDP glucuronyltransferase-1 polypeptide A1 (UGT1A1) was related to a significantly altered disposition of irinotecan and its metabolites, and was therefore associated with irinotecan-induced toxicity. This study focused on UGT1A1-mediated conversion of SN-38 to SN-38G, and systematically investigated the CsA-irinotecan interactions in vitro and in vivo. After treatment with 10 mg·kg-1 CsA for 7 days, the bile excretion of irinotecan and its metabolites decreased and AUC0-∞ increased significantly. The AUC0-∞ (SN-38G)/AUC0-∞ (SN-38) was significantly reduced when compared with that in vehicle-treated rats. In the liver microsome incubation system, the IC50 of CsA for UGT1A1 enzyme was 9.4 μM. Furthermore, the UGT1A1 mRNA and protein expression levels were significantly reduced. The present study indicated that CsA treatment could enhance the systemic exposure and toxicity of SN-38 by inhibiting the UGT1A1 enzyme. The inhibition of UGT1A1 enzyme might be a critical factor in the failure of CsA improving irinotecan's treatment index.