A high incidence of aortic rupture (46.8 per cent) resulted when turkeys were fed 0.07 per cent b-aminopropionitrile (BAPN), but few deaths (3.2 per cent) or none occurred when poults were fed 0.07 per cent BAPN with 0.03 or 0.1 per cent propranolol. There was no effect on mortality rate when the same level of BAPN was fed with 0.01 or 0.003 per cent propranolol. A significant depression of blood pressure and heart rate followed the feeding of BAPN with 0.1 per cent propranolol, but not at the other three dosage levels. Aortic tensile strength was greatest in turkeys fed the high level of propranolol with BAPN and least in poults fed the low level of the drug, with intermediate levels producing tensile strengths proportional to concentration of propranolol. The definitive mode of action of propranolol in the protection of turkeys from the development of BAPN-induced aortic rupture was not determined. It is suggested that the BAPN-fed turkey might be a suitable laboratory animal for screening drugs that appear to be potentially valuable for the treatment of aortic dissecting aneurysms in man. © 1968, SAGE Publications. All rights reserved.