Abstract: There are provided novel compounds of formula (I) wherein: A represents a five membered heterocyclic aromatic ring containing 1 to 3 heteroatoms which may be the same or different and are selected from O, N and S; or a six membered heterocyclic aromatic ring containing 1 to 3 nitrogen atoms; and R1, R2, R3 and R4 are as defined in the Specification and pharmaceutically acceptable salts thereof and enantiomers and tautomers thereof; together with processes for their preparation, compositions containing them and their use in therapy. The compounds are inhibitors of nitric oxide synthase and are thereby particularly useful in the treatment or prophylaxis of inflammatory disease and pain.