There are provided compounds of formula (I), ##STR1## wherein D represents a five membered heterocyclic aromatic ring containing 1 to 4 heteroatoms selected from O, N or S, optionally substituted at a carbon atom by halogen, trifluoromethyl, alkyl C1 to 6, nitro or cyano, and which is connected to the remainder of the compound of formula (I) through a carbon atom; A represents N(X) or CH(--(CH.sub.2).sub.m --NXY); U represents NH, O or CH.sub.2 ; V represents (CH.sub.2).sub.a ; W represents (CH.sub.2).sub.b ; a, b, m, X and Y are as defined in the specification, together with processes for their preparation and compositions containing them. Compounds of formula (I) are nitric oxide synthetase inhibitors and are useful in therapy.