A series of 4,4a,5,6- tetrahydro-4a-substituted-benzo(h)cinnolin-3(2H)-one (II a-e) (III a) were synthesized and evaluated pharmacologically in comparison with the 4a-unsubstituted congeners (I), which were reported to be potent antithrombotic and antihypertensive agents. All the test compounds, apart from (II e) (R2 = OCH3) and (III a) (R1,R2 = H), displayed significant antihypertensive activity. In particular (II a) (R1,R2 = H) was more active than the unsubstituted analogue (I a). Moreover (II d) (R1 = OCH3), (II e) (R2 = OCH3) and (III a) showed antithrombotic activity comparable to or higher than that of ASA.