The identification of the active pharmacophore required for potent inhibition of steroid sulphatase activity, i.e. an aryl-O-sulphamate structure, has led to the synthesis and testing of a large number of 1-4 ring-based inhibitors. 4-Methylcoumarin-7-O-sulphamate (COUMATE) was one of the first non-steroid based inhibitors identified. In an attempt to increase the potency of this class of inhibitor a series of tricyclic COUMATEs (665-6615 COUMATEs) have been synthesised and evaluated. Using placental microsomes as a source of oestrone sulphatase (E1-STS) the size of the third ring of the tricyclic COUMATEs was found to have a marked effect on inhibitor potency. Whereas 665- and 6615-COUMATEs had IC(50)s of 200 and 370 nM, respectively, the most potent inhibitor in vitro in this series was 6610 COUMATE with an IC(50) of 1 nM. Selected inhibitors were tested for their in vivo potency by administration of a single dose (0.1 or 1 mg/kg, p.o.) to female rats. Surprisingly, in vivo 6615 COUMATE proved to be the most active drug, inhibiting rat liver E1-STS activity by 23 and 94% when assayed 24 h after administration of the 0.1 and 1 mg/kg doses. E1-STS activity in brain tissue and white blood cells was also found to be inhibited when selected drugs were tested. These studies have identified a number of tricyclic COUMATEs with therapeutic potential.