3,4-Dihydro-2 H-1,4-benzoxazine derivatives possessing both thrombin inhibitory and glycoprotein IIb/IIIa (GPIIb/IIIa) receptor antagonistic activities were obtained by combining mimetics of the d-Phe-Pro-Arg pharmacophore of thrombin inhibitors and the Arg-Gly-Asp pharmacophore of GPIIb/IIIa receptor antagonists in the same low molecular weight peptidomimetic compound. Systematic variation of the position of substituents around the 3,4-dihydro- 2H-1,4-benzoxazine nucleus, the distance between the carboxylate and amidine moieties, together with additional substituents to fill the thrombin S 2 and S 3 pockets resulted in compounds displaying submicromolar inhibition constants ( K i) for thrombin and submicromolar IC 50 for inhibition of binding of fibrinogen to platelet GPIIb/IIIa receptor. Some of these compounds, such as 17a, 17b, 17d, and 17h possessing a well balanced activity at both targets, are a good starting point for further optimization. Incorporation of anticoagulant and platelet antiaggregatory activity in the same molecule constitutes a promising approach toward novel antithrombotic agents.