A series of 4-amino-7,8-dimethoxy-5H-pyrimido[5,4-b]indole derivatives has been synthesized. These compounds resemble carbazeram and other pyridazino compounds with activity in the cardiovascular system. Some of these new compounds possess inotropic activity (Table 2), with a complementary effect on the inhibition of different CGI-PDE (Table 3). The most active compounds 5, 6d, and 7 also possess activity as vasodilators (Table 4). Some of these new compounds inhibit blood platelet aggregation induced by ADP and AA and are active as inhibitors of human platelet PDEs (Tables 5 and 6).