The characteristics of platelet anti-aggregant activity of 2, 2-dimethylthiazolidine hydrochloride (I) and 2-(4-hydroxy-3-methoxyphenyl)thiazolidin (II) have been evaluated. Compounds I and II were potent inhibitors of collagen- and arachidonic acid-induced aggregation of rat and rabbit platelets (IC₅₀ or IC₁₀₀ values : 10^-5-10^-4 M), while they were less effective in the cases of adenosine 5'-diphosphate 2Na, A-23187 and labile aggregation-stimulating substance. Also, both compounds inhibited platelet shape change induced by a low concentation of arachidonic acid. Compounds I (100μM) and II (25μM) significantly inhibited serotonin release and thromboxane B₂ formation induced by arachidonic acid in rabbit platelets. Compound I (100μM) inhibited prostaglandin I₂ formation in rat aorta, but II (100μM) did not. In contrast, neither compound had any effect on platelet adhesiveness. These results strongly suggest that I and II prevented platelet aggregation, shape change and serotonin release reaction via their inhibitory effects on arachidonic acid metabolism of platelets, and the inhibiting potency of II was higher than that of I.