Carica papaya L. (papaya) seed preparations are used in traditional medicine to expel intestinal worms in human and ruminants. In the present study, an ethanol extract of papaya seeds (EEPS; 0.1–6.4 mg/mL) caused concentration-dependent inhibition of jejunal contractions in contrast to corresponding concentrations of DMSO (solvent control). The inhibitory effect of EEPS on jejunal contractions was significantly irreversible. Previous studies have indicated that benzyl isothiocyanate (BITC) is the main bioactive compound responsible for the anthelmintic activity of papaya seeds. In the present study, standard BITC (0.01–0.64 mmol/L) also caused significant irreversible inhibition of jejunal contractions. Recovery of jejunal contractions after BITC-induced inhibition was weaker than recovery after EEPS-induced inhibition (BITC versus EEPS: 19 ± 7% vs 38 ± 13%). Cryosections of the jejunum showed marked morphological damage of the segments treated with BITC in contrast to DMSO-treated segments. EEPS-induced jejunal damage was, however, less marked. These results indicate that papaya seed extract and BITC, its principal bioactive constituent are capable of weakening the contractile capability of rabbit isolated jejunum. It is thus envisaged that at the toxic level that will be needed to kill and expel intestinal worms in vivo, BITC may also cause impairment of intestinal functions. Copyright © 2005 John Wiley & Sons, Ltd.