Muscarinic receptor mediated membrane currents and contractions were studied in isolated canine colon circular smooth muscle cells. Carbachol (105M) evoked a slow transient inward current that was superimposed by a transient outward current at holding potentials greater than 50 mV. Carbachol contracted the cells by 70 ± 2%. The effects of carbachol were blocked by atropine (106M), tetraethyl ammonium (20 mM), and BAPTA-AM (25 mM applied for 20 min). The inward current and contraction were not sensitive to diltiazem (105M), nitrendipine (3 × 107M), niflumic acid (105M), or N-phenylanthranilic acid (104M), but were gradually inhibited after repetitive stimulations in Ca2+free solution. Ni2+(2 mM) inhibited the inward current by 67 ± 4%. The inward current reversed at +15 mV. The outward component could be selectively inhibited by iberiotoxin (20 nM) or by intracellular Cs+. Repeated stimulation in the presence of cyclopiazonic acid (CPA, 3 µM) inhibited the carbachol-induced outward current and partially inhibited contraction. CPA did not inhibit the inward current. In conclusion, muscarinic receptor stimulation evoked a CPA-sensitive calcium release that caused contraction and a CPA-insensitive transient inward current was activated that is primarily carried by Ca2+ions and is sensitive to Ni2+.Key words: calcium, carbachol, smooth muscle, cyclopiazonic acid, sarcoplasmic reticulum.