We studied the effects of berberine and Geranii Herba extract (GH) on different diarrheal models of mice, on the contractions of the isolated guinea pig intestinal smooth muscle and on the peristalsis in the rat intestine, comparing these effects with those of atropine (ATR) and papaverine (PAP). 1) Berberine significantly inhibited both the diarrhea induced by castor oil and that induced by BaCl2 at doses higher than 25 mg/kg, p.o., but did not inhibit diarrhea induced by pilocarpine or serotonin even at 250 mg/kg, p.o. GH exhibited a significant inhibition of BaCl2 induced diarrhea. ATR prevented pilocarpine-induced diarrhea most strongly in diarrheal models. PAP significantly inhibited castor oil-, BaCl2- and pilocarpine-induced diarrhea, but not the serotonin-induced one. 2) Berberine inhibited ACh- or Ba(2+)-induced contractions of the ileum and colon at a concentration of about 10(-5) g/ml, and PAP did so at about 10(-6) g/ml, but GH (10(-3) g/ml) failed to inhibit them. ATR prevented the ACh-induced contraction of the ileum and colon at concentrations 1/1000 of those needed to prevent the Ba(2+)-induced one. On the other hand, berberine as well as PAP exhibited a noncompetitive inhibition of the contractile response induced by ACh, whereas ATR showed a competitive inhibition. 3) Berberine, GH, ATR and PAP showed significantly inhibited the spontaneous peristalsis in the intestine. These results suggest that berberine and GH have antidiarrheal action and that the mechanism of action of berberine may be somewhat differ from that of GH.