The effects on an acute myocardial infarction of equipotent hypotensive doses of nitroglycerin and dipyridamole were studied using the thermographic technique in open-chest dogs. Nitroglycerin did not reduce the cool ischaemic area in any of the nine dogs studied. In six of the dogs, in which the ischaemic region could be properly measured, the cool area slightly grew in size. Dipyridamole reduced the size of the ischaemic area in all the 20 dogs used. In the 19 in which the size could be adequately digitalised, the decrease in infarction size was statistically significant.