Bile acids cause diarrhea by inducing colonic secretion, probably mediated through the cyclic AMP system. The aim was to determine the effects of an adenylate cyclase inhibitor, propranolol, on deoxycholic acid (DCA) stimulation of net secretion and the cyclic AMP system in the colon. In each of 30 New Zealand white rabbits, 0.9% NaC1 as control and 6 mM and 8 mM DCA were injected in random sequence into three colonic loops in situ. Propranolol, 4 mg per kg was administered intravenously to 12 of the 30 rabbits 1/2 hr before preparation of the loops, i.e., 5 1/2 hr before the rabbits were killed. In the 18 untreated animals, 6 and 8 mM DCA significantly stimulated colonic net secretion and mucosal adenylate cyclase activity; 6 mM DCA caused no change in mucosal phosphodiesterase activity, whereas 8 mM DCA caused a 25% decrease (P less than 0.01). In propranolol-treated animals compared to untreated animals, the volume of luminal fluid in controls was not different, with 6 mM DCA it was 88% less (P less than 0.01), and with 8 mM DCA it was 45% less (P less than 0.01); adenylate cyclase activity in controls was 43% less (P less than 0.01), with 6 mM DCA it was 67% less (P less than 0.01), and with 8 mM DCA it was 65% less (P less than 0.01); phosphodiesterase activity in controls and with 6 mM DCA was not different and with 8 mM DCA it was 38% greater (P less than 0.02). In conclusion, propranolol prevented DCA stimulation of colonic net secretion and inhibited the cyclic AMP system. Propranolol, therefore, warrants investigation as therapy for diarrhea caused by bile acids in the colon.