Effects of auxin polar transport inhibitors: methyl 2-chloro-9-hydroxyfluorene-9-carboxylate (morphactin IT 3456), 2,3,5-triiodobenzoic acid (TIBA) and N-1-naphthylphthalamic acid (NPA) on elongation growth of the excised fourth internode of tulips (Tulipa gesneriana L.) cv. Apeldoorn were studied. After removal of flower bud, the continuing elongation of the excised fourth internode kept in water in normal or inverted position was observed, the elongation in inverted position being significantly higher than that in the normal position. On the other hand, the application of these inhibitors (0.2%, w/w in lanolin) in the place of removed flower bud substantially enhanced the elongation, the stimulation being much higher in the normal position than in the inverted one. When the inhibitors were applied in the middle or 1 cm from the base of the internode, the growth-promoting effect of these inhibitors was observed both in the upper and lower part of the internode, being greater in the upper part of the internode, regardless of the position of explants. Simultaneous application of indole-3-acetic acid (IAA) (0.1%, w/w in lanolin) at the place of the removed flower bud in the normal position with morphactin, TIBA and NPA applied in the middle or 1 cm from the base of the excised internode greatly stimulated the elongation, whereas almost no growthpromoting effect of these inhibitors was observed, in comparison to IAA. On the contrary, when IAA was applied on the base of the excised internode in the inverted position, the growth was inhibited, compared to that with lanolin only. The inhibitory effect of auxin was eliminated by the simultaneous application of morphactin, TIBA and NPA placed in the middle of the excised internode. These results suggest that auxin levels in the excised internode regulated by auxin polar transport play a crucial role in the regulation of its elongation growth.