A method for the preparation of 6-alkyl-2-aryl-2H-pyrazolo[4,3-d]pyrimidine-5,7(4H,6H)-diones in reaction of ethyl 1-aryl-4-isocyanatopyrazole-3-carboxylates with aliphatic amines followed by cyclization of the obtained 4-ureidopyrazole-3-carboxylic acid esters in the presence of bases is proposed.