We examined the involvement of cAMP-dependent protein kinase (A kinase) 2 in the inhibition by cilostamide, a specific inhibitor of the low K m cAMP-phosphodiesterase (PDE), on 9, 11-epithio-11, 12-methanothromboxane A 2 (STA 2)-induced platelet aggregation. For comparative purposes, the PGE 1 analogue, 17 S-20-dimethyl- trans- Δ 2-PGE 1 (OP-1206) was used. OP-1206 ( IC 50 = 18 ± 0.55 nM) and cilostamide ( IC 50 = 40 ± 4.5 nM) were both potent inhibitors of the platelet aggregation induced by STA 2 (1 μM). OP-1206 and cilostamide dose-dependently inhibited elevations in intracellular free Ca 2+ ([Ca 2+] i) caused by STA 2. OP-1206 caused an almost complete inhibition of Ca 2+ mobilization, but cilostamide did not prevent the STA 2-induced elevation in [Ca 2+] i to the same extent as OP-1206, even at a high concentration (> 200 nM). Cilostamide did not increase the cAMP level at concentrations (5–100 nm) which affected STA 2-induced aggregation. OP-1206 significantly increased cAMP contents in platelets, and the degree of aggregation inhibition by Op-1206 appears to be related to the size of increase in cAMP. OP-1206 increased phosphorylation of the 50,000 mol. wt vasodilator-stimulated phosphoprotein, at concentrations of 7.9–79 nM, which inhibited aggregation induced by STA 2. Cilostamide treatment resulted in a marginal increase in the 50,000 mol. wt phosphorylation at concentrations (10–100 nm) which completely inhibited the STA 2-induced aggregation. (8R ∗, 9S ∗, 11S ∗)-(−)-9- Hydroxy-9-n- hexyloxy-8- methyl-2,3,9,10- tetrahydro-8,11- epoxy-1H, 8H, 11H-2, 7b , 11 a-triazadibenzo( a, g)-cytoocta( c, d, e)trinden-1-one (KT-5720), a specific inhibitor of A kinase, not only reversed the inhibition by OP-1206 of STA 2-induced platelet aggregation, but also inhibited the OP-1206-induced protein phosphorylation. However, the inhibition by cilostamide of STA 2-induced aggregation was not prevented by pretreatment with KT-5720. Inhibition of the STA 2-induced aggregation by OP-1206 may be associated with cAMP-dependent protein phosphorylation, while cilostamide may have inhibitory effects on STA 2-induced platelet activation through mechanisms other than the activation of A kinase.