The use of medicinal herbs among the general population gives rise to the possibility of therapeutic or toxic effects in patients that use these plants. The effects of Achillea millefolium hydroalcoholic extract on the contractile responses of the isolated guinea-pig ileum were investigated. The effect of the Achillea millefolium extracts at five concentrations ranging form 0.05 to 5 mg mL(-1) was tested. The terminal ileum was removed. Segments were fixed in an organ bath containing in Tyrode solution. Contraction changes in the tissues were monitored using force displacement transducer amplifier connected to physiograph. Each segment served as its own control. Achillea millefolium inhibited the contractile response in a dose-dependent manner. The 50% effective concentration values (EC50) were calculated, which was 1.5 mg mL(-1). Regression analysis had shown that with increasing in extract concentration the effect of extract was increased. The coefficient of extract dose was 0.031 mg. Present results demonstrate that extract prepared from the plant of Achillea millefolium inhibited electrical induced contractions of the guinea-pig ileum when tested in vitro. This effect is dose dependent and reversible.