Arterial access is an integral part of modern neonatal intensive care. Traditionally, umbilical arterial lines have been utilized for this purpose but carry a significant risk of major thromboembolic complications. While peripheral arterial lines are considered to be safer, they have a limited lifespan secondary to vasospasm, intimal damage and thrombus formation. Tolazoline is a potent vasodilator which has primarily been used in the NICU to treat persistent pulmonary hypertension of the newborn. In the past year we have utilized low dose intraarterial tolazoline infusions [0.02 - 0.2 mg/kg/hr] to treat 5 critically ill infants [26-41 weeks gestation, birth weights 450-3880 grams] with malfunctioning arterial lines, as evidenced by waveform dampening and the inability to withdraw blood. In all patients arterial access was very difficult to achieve; in two cases the infants were extremely small for gestation, while in the remaining 3 cases the infants were profoundly acidotic secondary to asphyxia, diaphragmatic hernia and idiopathic PPHN respectively. Vasospasm resolved within 5 minutes of initiating the tolazoline infusion in all 5 patients. One infant had recurrence of vasospasm after 6 hours which did not respond to an increased dose of tolazoline. Three infants had their infusions continued for 2 to 14 days until their lines were electively discontinued. One infant had the tolazoline infusion weaned and discontinued after 48 hours with the line remaining patent for 15 days. No hypotension was experienced with repetitive blood gas sampling and catheter flushing in any of our patients. We conclude that tolazoline is useful in alleviating vasospasm in malfunctioning arterial lines in critically ill neonates. A randomized trial is currently underway to evaluate the most appropriate dosage and duration. Supported by Canadian Lung Association and Children's Hospital of Winnipeg Research Foundation.