1. We have proposed that contractile tension of the ductus arteriosus is sustained by a cytochrome P450-linked mechanism acting as a limiting step in the synthesis of endothelin-1 (ET-1). In the present study, we have used the isolated ductus from near-term foetal lambs and guinea-pigs to investigate the effect on both muscle tone and ET-1 formation of 1-aminobenzotriazole (ABT), a suicide substrate for mono-oxygenase reactions. 2. ABT relaxed the lamb ductus at rest (2.5% O2) and during the oxygen contraction (15 to 95% O2). The effect was seen at 40 microM, and at 0.8 mM active tone was almost completely abolished. ABT (1 mM) also reversed the oxygen contraction in the guinea-pig ductus. 3. In the lamb ductus, the ABT response was not affected by removal of the endothelium or by treatment with 2.8 microM indomethacin (at 2.5% O2) and the ensuing contraction. 4. At both low and high concentration, ABT relaxed marginally, or not at all, the potassium-contracted (55 mM) ductus from either species. 5. ET-1 release from either the intact or endothelium-denuded lamb ductus tended to decrease in the presence of ABT (1 mM), whilst during the same treatment cyclic GMP content of the tissue remained unchanged. 6. We conclude that ABT relaxation is due to suppression of a contractile mechanism and not to activation of prostaglandin- and NO-mediated relaxing mechanisms. This contractile mechanism has a cytochrome P450-based mono-oxygenase reaction as a key component.