Abstract: There are provided novel compounds of formula (I) ##STR1## wherein A represents a benzo ring; a six membered heterocyclic aromatic ring containing 1 to 3 nitrogen atoms; or 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; and R.sup.1, R.sup.2, R.sup.3 and X 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 and prophylaxis of inflammatory disease and pain.