Effects of caerulein on intestinal motility have been studied in comparison with those of neostigmine, pantethine, prostaglandin E1 (PGE1) and prostaglandin F2alpha (PGF2alpha). Caerulein facilitated electric discharges in the intestinal tracts of anaesthetized rabbits and exhibited a greater potency than either neostigmine or pantethine. The small intestine was more sensitive to this agent than was the large intestine. PGE1 inhibited while PGF2alpha facilitated electric discharges in the small intestine. A complete inhibitory effect of the excitatory of caerulein was not demonstrated with atropine. Caerulein promoted the transit of charcoal meal through the intestine of the mouse and was approximately 30 times more potent than was neostigmine. At high doses, the promotion was reduced and the reduction was inhibited by reserpine or phentolamine-propranolol. Our observations indicate that caerulein produces a catecholamine releasing action in high doses. Caerulein promoted the transit in the cecectomized mice at doses 30 times larger than given to intact mice. Caerulein, neostigmine, PGE1 and PGF2alpha produced an excitatory effect on the isolated intestine of the rabbit. Minimal effect concentrations were 3 X 10(-10 approximately 10(-9), 10(-8), 10(-9) and 3 x 10(-11) g/ml in the ileum and 3 X 10(-9) approximately 10(-8), 10(-8) approximately 3 x 10(-8), 10(-10) and 10(-11) g/ml in the proximal colon, respectively.