BACKGROUND Left internal mammary artery spasm is well recognized during coronary artery bypass operations. Papaverine has been used by many surgeons to maximize mammary artery flow perioperatively, but the best delivery method is not known. We analyzed two techniques used at our institution. METHODS Fifty-eight patients were randomized into three groups to compare papaverine's ability to prevent spasm and to treat established spasm. Group 1 was control and no treatment was employed. In group 2, papaverine was injected with a blunt needle through the endothoracic fascia parallel to the mammary artery before harvest to assess spasm prevention. In group 3, papaverine was delivered perivascular in an identical manner to group 2 but after the mammary artery was dissected from the chest wall. This group was an evaluation of spasm treatment. Drug dosage was the same for both groups and routine bypass grafting was performed. Before anastomosing the mammary artery to the left anterior descending artery, blood flow was recorded for 15 seconds and flow per minute calculated. Cardiopulmonary bypass pressures were maintained at 70 mm Hg during collection. RESULTS Mean blood flows were: group 1 = 86.2 mL/min, group 2 = 122.5 mL/min, and group 3 = 139.7 mL/min. Left internal mammary artery flow in group 3 was statistically different from control (p = 0.0457). Group 2 flow approached but did not reach statistical significance (p = 0.0874). Mammary artery dissection times for the three groups were not different. CONCLUSIONS Papaverine delivery to the left internal mammary artery after dissection treats spasm effectively, improves blood flow at the time of its anastamosis to the left anterior descending artery, and avoids any risk of intimal injury. Injection of papaverine before mammary artery harvest does not shorten dissection time, and flow is not statistically improved.