“…fentanyl and alfentanyl, two potent narcotic agonists (Gardocki & Yelnosky, 1964;Niemegeers & Janssen, 1981) (Pert et al, 1976;McKnight et al, 1983). Fentanyl, alfentanyl and dynorphin A (1-13) were essentially equipotent in causing contraction of the mouse colon and, as in the guinea-pig ileum, were more potent than morphine (Fennessy et al, 1969, Yoshimura et al, 1982. The pA2 values obtained in the mouse colon for naloxone antagonism of morphine, [Met5]enkephalin and dynorphin A (1-13) were not equivalent.…”