BACKGROUND--Nitric oxide is a potent dilator of the pulmonary vasculature. There have been no previous reports on the action of nitric oxide on the arterial duct. OBJECTIVES--To determine the responses of isolated post-term arterial duct rings from lambs to oxygen, prostaglandin E2 (PGE2) and the nitric oxide donor, 3-morpholinosydnonimine (SIN-1). SETTING--Experimental laboratory. SUBJECTS--Six neonatal lambs. METHODS--Lambs aged 1-5 days were killed and the arterial duct and aorta excised and cut into rings. These were mounted on tension gauges in organ baths containing Krebs-Henseleit solution. Rings were exposed to increasing concentrations of oxygen, PGE2 and after preconstriction with potassium (40 mmol/l) to SIN-1. Tension and relaxation responses were recorded. RESULTS--Increased oxygen tension resulted in increased tension in the ductal rings above 88.9 mm Hg as previously described. No response to PGE2 occurred before or after ductal rings were exposed to oxygen. SIN-1 caused relaxation of smooth muscle in the arterial duct to a similar degree as that in the aortic rings. CONCLUSIONS--As previously shown, oxygen is a potent constrictor of the arterial duct. The post-term arterial duct does not relax in response to PGE2 possibly as a result of inactivation by oxygen of the special sensitivity of the duct to PGE2. SIN-1 is a potent smooth muscle relaxing agent in the term arterial duct and may have a role in the initial management of neonates with duct dependent pulmonary circulation.