The effects of aminophylline were examined in 19 conscious dogs subjected to coronary arterial occlusion. Measurements were made of left ventricular pressure and its first derivative (dP/dt), segment length and the velocity of segment length shortening in normal an severely ischemic zones. Regional blood flow was measured in these zones using the radioactive microsphere technique. Coronary occlusion increased heart rate, mean arterial pressure and left ventricular end-diastolic pressure but did not change left ventricular systolic pressure or dP/dt significantly. It also resulted in increased end-diastolic segment length and reduced segment length shortening (114 +/- 6 percent, that is, paradoxical bulging) associated with marked reduction of blood flow to ischemic myocardium. Aminophylline, 1 mg/kg per min for 9 to 15 minutes administered after occlusion, increased heart rate 6 +/- 2 beats/min, mean arterial pressure 5 +/- 1 mm Hg, left ventricular systolic pressure 9 +/- 2 mm Hg and left ventricular dP/dt 670 +/- 50 mm Hg/s while reducing left ventricular end-diastolic pressure by 3.4 +/- 0.3 mm Hg. In severely ischemic zones aminophylline increased transmural blood flow by 21 +/- 8.0 percent (p less than 0.02), reduced end-diastolic segment length by 0.23 +2- 0.05 mm (p less than 0.01) and reduced paradoxical bulging by 0.15 +/- 0.06 mm (p less than 0.02). Thus, in the presence of coronary arterial occlusion, aminophylline increased mean arterial pressure, left ventricular dP/dt and heart rate while reducing left ventricular end-diastolic pressure. In severely ischemic myocardium aminophylline appeared to exert a salutary effect and improved both regional perfusion and function.