This invention relates to the discovery of 3-and 5-substituted analogues ofthe selective platelet lowering agent anagrelide with reduced potential forcardiovascular side-effects which should lead to improved patient complianceand safety in the treatment of myeloproliferative diseases. More specifically,the present invention relates to certain imidazoquinazoline derivatives whichhave utility as platelet lowering agents in humans. The compounds of the presentinvention function by inhibiting megakaryocytopoeisis and hence the formationof blood platelets.