Diarrhea is one of the main causes of infant mortality in Ethiopia and in many other developingrncountries, causing about 4 to 8 million deaths annually worldwide. Despite recent reductions inrnmorbidity and mortality worldwide, diarrhea still remains one of the leading causes of death inrndeveloping countries. Like many other developing countries, people in Ethiopia strongly rely onrnthe therapeutic benefits of traditional medicine to treat diarrhea. One such plant is Thalictrumrnryhnchocarpum Dill. & A. Rich. Thus, in the present study, the 80% methanol extract of the rootrnof T. ryhnchocarpum and the compound isolated thereof have been evaluated for theirrnantidiarrheal activity. Phytochemical investigation of the 80% methanol root extract of T.rnryhnchocarpum by preparative TLC over silica gel resulted in the isolation of one major alkaloidrnunequivocally identified as berberine using various spectroscopic techniques, including MS, rn13rnC-NMR and DEPT spectral data.rn Both the total extract and berberine were tested for their in vivo antidiarrheal activity on threernmodels namely, castor oil-induced diarrhea, charcoal meal and enteropooling models in mice. Itrnwas shown that both the extract and berberine possess significant antidiarrheal activity in a dosedependentrnrnmanner on all the tested models. Particularly, the root extract showed the maximumrnactivity on castor oil-induced diarrhea model, with 71.4%, 74.0% and 78.0 % (P < 0.01) andrnberberine displayed even higher activity with 81.0%, 87.6 % and 88.3% at tested concentrationrnof 100, 200 and 400 mg/kg, respectively. Antidiarrheal activity of the root extract and berberinernwas also noted on the other two models. When administered up to a dose of 2000 mg/kg, neitherrnthe root extract nor berberine showed acute toxicity in Swiss albino mice. In view of the presentrnresults, it can be concluded that the root extract of T. ryhnchocarpum and berberine could servernfor the treatment of diarrhea thereby supporting the traditional claim of the plant againstrndiarrhea.