The trypsin-like serine protease factor Xa, located at the convergence point of the intrinsic and extrinsic coagulation cascade, is responsible for thrombin generation and has emerged as an attractive target for the development of a novel type of anticoagulants. Results obtained with parenteral inhibitors indicated that factor Xa is probably a more safe and selective target than the clotting protease thrombin. Modification of the known lead structure benzylsulfonyl-D-Ser(tBu)-Gly-4-amidinobenzylamide led to inhibitors with D-homophenylalanine (D-hPhe) as P3-residue. These inhibitors have a significantly improved affinity as factor Xa inhibitors (inhibitor 802: Ki = 6.0 nM), however, they were rapidly eliminated from the circulation of rats. The present work describes the further optimization of these analogues by the replacement of D-hPhe in P3 position with other non-natural aromatic homoamino acids in combination with different P4 and P2 residues to improve their affinity and selectivity as factor Xa inhibitors. In addition, the work was focused on the improvement of the pharmacokinetic behaviour of these derivatives. The most potent and selective inhibitor 1332 contains a D-2-homopyridylalanine-N-oxide as P3-residue and inhibits factor Xa with a Ki-value of 0.32 nM. Inhibitor 1332 shows a prolonged elimination half life in rats. This compound is highly effective in the standard clotting assays (aPTT, PT) and reduces the rate of thrombin generation. A very strong efficacy of this analogue could also be demonstrated in a rat model of venous thrombosis. To improve the poor oral bioavailability found for the benzamidine-derived inhibitors, a hydroxyamidino-prodrug of compound 1332 was synthesized. However, also the prodrug 1331 (benzylsulfonyl-D-2-homopyridylalanine-N-oxide-Gly-4-hydroxyamidinobenzylamide) has only a low oral bioavailability below 5 % in rats at a dose of 5 mg/kg.