A series of 7, 8-fused heterocyclic tetrahydroisoquinolines were synthesized and tested as inhibitors of rabbit adrenal phenylethanolamine-N-methyltransferase (PNMT). 6, 7, 8, 9- Tetrahydro [1, 2, 3] thiadiazolo [5, 4-h] isoquinoline 5 (SK&F 86607) was found to be a potent inhibitor of PNMT with an ICN similar to that of 7, 8-dichloro-1, 2, 3, 4-tetra-hydroisoquinoline (1, SK&F 64139). The isomeric tetrahydro [1, 2, 3] thiadiazolo [4, 5-h]- and tetrahydro [1, 2, ...