Isocarbacyclin methyl ester (CAS 88931-51-5, TEI-9090) and its free acid (CAS 88911-35-7, TEI-7165) are chemically stable analogues of prostaglandin (PG) I2. The cAMP increasing activities of TEI-9090 and TEI-7165 were investigated. TEI-9090, TEI-7165, PGI2, and PGE1 caused the accumulation of cAMP in rabbit and human platelets. The effects of TEI-9090, TEI-7165, and PGE1 were longer lasting than the effect of PGI2. Each drug concentration-dependently increased the cAMP level in rabbit and human platelets. The order of cAMP-increasing activity was PGI2 > PGE1 = TEI-9090 = TEI-7165 in rabbit platelets and PGI2 > TEI-9090 = TEI-7165 > PGE1 in human platelets. The antiplatelet aggregation activity of TEI-9090 in rabbit and human platelets was remarkably decreased by the addition of an esterase inhibitor, diisopropyl fluorophosphate (DFP). However, this activity of TEI-7165 was not affected by the inhibitor. In the presence of DFP, the cAMP accumulation by TEI-9090 in rabbit platelets was almost completely inhibited but that by TEI-7165 was not inhibited. These results suggest that TEI-9090 is deesterified to TEI-7165 and that this free acid then inhibits platelet aggregation via an increase in the cAMP level in the platelets.