Novel fibrinogen receptor antagonists of the formula: X--Y--Z-Aryl-A--B and the pharmaceutically acceptable salts thereof, wherein Aryl is a 5-membered aromatic ring system containing 1 or 2 heteroatoms chosen from N, O or S wherein ring atoms may be unsubstituted or substituted with R.sup.5 and wherein S may be substituted with 0, 1 or 2 oxygen atoms; and X comprises various Nitrogen substituents including aromatic or nonaromatic systems; Y comprises, for example, alkyl or alkylaminocarbonylalkyl groups and Z and A are substituents which when present are independently chosen from alkyl or alkyloxyalkyl or other groups as defined herein; B is (a) or (b). The claimed compounds exhibit fibrinogen receptor antagonist activity, inhibit platelet aggregation and are therefore useful in modulating or preventing thrombus formation.