Galactinol synthase (GS, UDP-alpha-d-galactose:1l-myo-inositol-1-O- alpha-d-galactopyranosyltransferase) is a key enzyme in the biosynthetic pathway of the raffinose family of oligosaccharides. The subcellular location of GS was studied in the parenchyma of stachyose-storing tubers of Japanese artichoke (Stachys sieboldii) by isolation of protoplasts and vacuoles. A comparison of the activities of GS, malate dehydrogenase, and alcohol dehydrogenase (extravacuolar markers) and alpha-mannosidase and beta-N-acetylglucosaminidase (vacuolar markers) in parenchyma protoplasts with those of vacuoles isolated from them showed that GS was an extravacuolar enzyme.