In decapitated Avena coleoptiles the regeneration of the physiological tip is inhibited by glucose, galactose, lactose and raffinose. Galactose is the strongest inhibitor, and a further analysis of its action indicates that it inhibits auxin synthesis. The galactose effect is not antagonized by other sugars. Sucrose, maltose, fructose, rhamnose and ribulose have no influence on the regeneration.