Bicyclic piperazinone based thrombin inhibitors of general structure 2 were prepared and evaluated in vitro and in vivo. These inhibitors, having in common an electrophilic basic trans-cyclohexylamine P-1 residue, displayed high thrombin affinity, high selectivity against trypsin and good in vivo efficacy in the rat arterial thrombosis model. (C) 2001 Elsevier Science Ltd. All rights reserved.