The antidiarrhoeal activity of six Egyptian medicinal plant extracts (200 and 400 mg kg(-1)) and their effect on motility of isolated rabbit's duodenum was investigated. Phytochemical screening of the plant extracts for their active constituents was also carried out by TLC. Oral administration of methanol extract from Conyza dioscoridis (CD) or Alhagi maurorum (AM) in a 200 mg kg(-1) dose exhibits a significant antidiarrhoeal effect against castor oil-induced diarrhoea, while Mentha microphylla (MM), Convolvulus arvensis (CA), Conyza linifolia (CL) produced no significant effect. In a dose of 400 mg kg(-1), Mentha microphylla, Conyza dioscoridis, Alhagi maurorum, Zygophyllum album (ZA), and Conyza linifolia produced a significant (P<0.01) effect, while Convolvulus arvensis produced no antidiarrhoeal effect in rats. Methanol extract of Mentha microphylla, Conyza dioscoridis, Zygophyllum album, and Convolvulus arvensis induced a dose-dependent (0.4-2.8 mg ml(-1)) relaxation of rabbit's duodenal smooth muscle. Alhagi maurorum and Conyza linifolia increased the contractile force in concentrations between 0.4 and 1.6 mg ml(-1). Higher concentrations (>3.2 mg ml(-1)) caused a rapid depressant effect. The depressant effect induced by Alhagi maurorum (in a higher dose) and Zygophyllum album appeared to be due to calcium channel blocking effect, since CaCl(2) could not restore the contractile response of the tissue impregnated in calcium free-medium. However, a ganglionic blocking effect appeared to be a possible mechanism of action of Mentha microphylla and Conyza dioscoridis since a stimulant dose of nicotine could not restore the contractile response of the tissue. The effect of Convolvulus arvensis and Conyza linifolia was not through any of the common mediators. Phytochemical screening revealed the presence of tannins, flavonoids, unsaturated sterols/triterpenes, carbohydrates, lactones and proteins/amino acids as major constituents.