A series of 1-(4,5-dihydro-1H-imidazol-2-yl)indole derivatives was prepared in order to evaluate their antiaggregatory activity in human platelets. The compounds 4a-m were prepared by reacting N-aryl-N-(4,5-dihydro-1H-imidazol-2-yl)hydroxylamines (2a-d) with monosubstituted acetylene derivatives 3a-b. Imidazoline derivatives 4 were further acetylated or sulfonylated to give amides 5a-c and sulfonamides 6a-c and 7a-c, respectively. Eight compounds were taken as representative aryliminoimidazoline analogs. Among them only one, 4m, showed a good concentration-dependent action against the primary or alpha 2-adrenoreceptor mediated phase of noradrenaline-induced aggregation in platelets.