1. We reported that (+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide (FK409) released nitric oxide (NO) spontaneously with a chemiluminescence analyzer. The aim of this study was to compare antiplatelet activities of FK409, a new NO releaser, with those of isosorbide dinitrate (ISDN) in vivo and in vitro. In order to elucidate the differences in antiplatelet activities between FK409 and ISDN, we compared their modes of action. 2. In in vitro experiments, FK409 had a more potent inhibitory effect on rat platelet aggregation induced by adenosine 5'-diphosphate (2.0 microM) than ISDN (IC50 = 4.32 +/- 0.95 microM and > 100 microM respectively). 3. In the rat extracorporeal shunt model (in vivo experiments), FK409 suppressed thrombus formation dose-dependently from 0.32 mg kg-1, p.o. and showed the maximum inhibition (52% inhibition vs. vehicle treatment) at 10 mg kg-1, p.o., while ISDN showed no inhibition at 10 mg kg-1 and only 17% inhibition at 32 mg kg-1, p.o. 4. FK409 could generate nitrite, which is an oxidative product of NO, much faster than ISDN in phosphate buffer solution and rat plasma during 60-min incubation at 37 degrees C. 5. These data show that FK409 has more potent antiplatelet effects than ISDN, by acting through spontaneously released NO.