Abstract Starting with ethyl 3-aminoindole-2-carboxylate, the synthesis of 3-amino-5H- pyrimido [5, 4-b] indole-2, 4-dione 5, 3-amino-5H-pyrimido [5, 4-b] indol-4-one 10 and some related compounds is described. Preliminary results about the inhibition of platelet aggregation by these compounds is reported.