In clinical use, irinotecan hydrochloride (CPT-11; 7-ethyl-10-[4-(piperidino)-1-piperidino]carbonyloxycamptothecin), a novel antitumor agent, causes a relatively high incidence of severe forms of diarrhea. We investigated whether baicalin, an inhibitor of beta-glucuronidase, which deconjugates the glucuronide of the active metabolite of CPT-11, SN-38 (7-ethyl-10-hydorxycamptothecin), and Japanese herbal medicines (Kampo medicines) which contain baicalin can ameliorate CPT-11-induced intestinal toxicity in rats. CPT-11 (60 mg/kg i.v. once daily for 4 consecutive days) induced intestinal toxicity characterized by diarrhea, loss of body weight, anorexia and disruption of intestinal epithelium. Treatment with baicalin (25 mg/kg p.o. twice daily) or Kampo medicines (TJ-14 and TJ-114; 1 g/kg p.o. twice daily) from the day before to 4 or 10 days after the start of CPT-11 administration resulted in significantly decreased weight loss, improved anorexia and delayed onset of diarrheal symptoms. Histological examination revealed that Kampo medicine-treated animals had less damage to the intestinal epithelium and that damage was repaired more rapidly than in control rats. These results suggest that the prophylactic use of Kampo medicines (TJ-14 and TJ-114) may be of value against CPT-11-induced intestinal toxicity.