1. The effects of several ions, Mn2+, Zn2+, Ba2+ and Ca2+, on spontaneous motility were investigated in longitudinal smooth muscle strips from sheep duodenum, in vitro. 2. Mn2+ (0.5-1.5 mM) and Zn2+ (0.5-5 mM) inhibited both the amplitude and frequency of motility in Krebs solution and in Ca(2+)-free medium. 3. Ba2+ (0.5-5 mM) evoked three types of contractile responses: (i) an increase in the frequency and a reduction of the amplitude of spontaneous contractions; (ii) a slight increase in muscle tone of the phasic contractions; and (iii) a rapid initial phasic contraction followed by slowly fading contraction. Ca2+ induced two kinds of responses in spontaneous motility: (i) a fast phasic contraction, followed by an increase in the amplitude and frequency of phasic contractions with no changes in its tone; and (ii) an increase in the amplitude of contractions. 4. The Ba(2+)-induced contractions were inhibited by EDTA, verapamil and diltiazem, but were not modified by sodium nitroprusside. The Ca(2+)-induced contractions were reduced by verapamil and diltiazem. 5. Our results show that Mn2+ and Zn2+ behave as inhibitors of sheep duodenum motility. In contrast, Ba2+ and Ca2+ stimulate motility. It is suggested that Ba2+ can penetrate the cells through voltage-dependent Ca2+ channels and behave as a partial substitute for Ca2+.