1. Eugenol (3-methoxy-4-hydroxy-propenylbenzene) or sodium eugenol acetate (4-O-acetic acid sodium-3-methoxy-1-propenylbenzene) (0.25, 0.5, 1 mM) concentration-dependently inhibited arachidonic acid (AA)-, collagen-, epinephrine- and ADP-induced platelet aggregation. 2. Eugenol or sodium eugenol acetate inhibited collagen-induced aggregation of washed rabbit platelets synergistically with creatine phosphate/creatine phosphokinase (CP/CPK, 5 mM/10 U/ml) or p-bromophenacyl bromide (p-BPB, 10 microM), and they also potentiated the inhibitory action of imidazole (0.5 mM) on AA-induced aggregation. 3. Eugenol or sodium eugenol acetate (0.25, 0.5, 1 mM) concentration-dependently inhibited AA-induced thromboxane B2 and prostaglandin E2 formation. 4. The rise of intracellular Ca2+ caused by collagen, epinephrine, ADP, and AA were inhibited by eugenol or sodium eugenol acetate (1 mM).