Inhibition of platelet aggregation was found in the essential oils extracted from the various parts of upland wasabi (Wasabia japonica forma terrestris "isauti). The antiplatelet activity increased in the order of leaves, rhizomes, petioles, and roots. To examine the inhibitory principles in upland wasabi oil, components in each part of the essential oil were identified and measured. Most of the volatile components in upland wasabi oils were m-alkenyl isothiocyanates and m-methylthioalkyl isothiocyanates, allyl isothiocyanate being the main component in every oil. Essential oils from roots and petioles contained rather large amounts of m-methylthioalkyl isothiocyanates. Although every authentic isothiocyanate inhibited platelet aggregation, m-methylthioalkyl isothiocyanates were more inhibitory, which explained the stronger inhibitory effects of essential oils from roots and petioles on platelet aggregation.