Abstract: New thrombin inhibiting N.sup..alpha. -arylsulfonyl-p-guanidinophenylalanine amides of the formula ##STR1## wherein Ar is a substituted aryl and R.sub.1 and R.sub.2 are an alkyl group having 1-5 carbon atoms or together with the amine nitrogen form an heterocyclic ring, in racemate form as well as in form of optical active antipodes, theirs pharmaceutically acceptable salts, methods for their preparation, pharmaceutical composition and diagnostical preparation containing these compounds, use of the compounds in treatments of thrombosis and methods of treatment of thrombosis as well as methods for determination of thrombin concentration in blood.