Superfusion of twelve human Taenia Coli strips with cholecystokinin (CCK) (Boots and GIH Karolinska), CCK-octapeptide and glucagon at doses of 25–1,000 mIDu/ml evoked muscular contractions. Dose-dependent response to acetylcholine (ACH) (0.15–1.0 μg/ml) was not altered by an underlying contraction induced by CCK, CCK-octapeptide or glucagon. Though atropine (0.1 μg/ml) abolished ACH effects, hormonal responses were unaffected. The muscle’s sensitivity was highest to CCK (Boots) and lowest to CCK-octapeptide, and the sigmoid colon was more sensitive than other parts of the bowel to both. Addition of glucagon had no effect on CCK-induced (Boots) responses. The results suggest (1) CCK and CCK-octapeptide act directly on the Taenia coli; (2) biologically active impurities in the whole CCK preparations explain their apparent increased activity; (3) glucagon does not block ACH or CCK, nor inhibit, directly, muscle contractions; (4) atropine (0.1 μg/ml) does not interfere with CCK or CCK-octapeptide on coloni