Effects of sucrose and cytokinin (trans-zeatin riboside [t-ZR]) on the formation of tuberous root of sweetpotato (Ipomoea batatas (L.) Lam.) were studied by directly applying these substances to young roots. In the roots that originally contained approximately 9 μg of endogenous sucrose per milligram of fresh weight (FW), further increase in the sucrose level did not necessarily result in tuberous root formation, whereas t-ZR application invariably induced tuberous root formation. However, t-ZR did not induce morphogenesis when the sucrose level in the root was lowered to approximately 2 μg mg^-1 of FW. Our results imply that high levels of endogenous sucrose are necessary for tuberous root formation and that in the presence of high sucrose concentrations, cytokinins can trigger tuberous root formation.