Intestinal mucosa heparin (IMH) and beef lung heparin (BLH) were infused into dogs subjected to myocardial ischemia by intermittent coronary artery occlusions. The IMH was from a mixture of beef, sheep, and pig intestinal mucosa. Initial control occlusion and recovery was followed by a second occlusion with 60,000 units of IMH or BLH added. Electrocardiographic S-T segment elevations (ST) were measured acutely. There were no significant differences in ST in non-ischemic myocardium before occlusions or with occlusions. In ischemic myocardium, IMH significantly lowered control ST 84% in amount (t = 6.1 p less than 0.00005), and 76% in number (t = 11.6 p less than 0.00001). BLH lowered control ST a significant, lesser, 36% in amount (t = 3.6 p less than 0.008), and 35% in number (t = 3.2 p less than 0.01). The difference between IMH and BLH in ischemic myocardium was a significant adenosine triphosphate (ATP) levels were assayed after 90 min. ATP levels were 31% higher in both ischemic and non-ischemic myocardium in IMH-treated dogs than in BLH-treated. It was concluded that IMH and BLH are functionally different, and IMH was significantly more effective.