We examined the mode of action of 4,4,4-trifluoro-3-(indole-3-)butyric acid (TFIBA) on rice seminal root elongation, the most effective application method of TFIBA to plants and the effect of TFIBA on ion uptake by rice seedlings. Elongation of the seminal root of rice was promoted approximately 80% by a continuous supply of 10−5 mol L−1 TFIBA, whereas elongation was inhibited approximately 40% by 10−4 mol L−1 TFIBA. The epidermal and cortical cells at 5 mm and 30 mm from the root apex were vertically elongated by 60% and 30%, respectively, by a continuous supply of 10−5 mol L−1 TFIBA. The large vertical elongation of root cells apparently resulted in 15% thinning of the root diameter and inhibition of root hair formation. When rice seeds were soaked in 10−5 mol L−1 or 10−4 mol L−1 TFIBA for 48 h during germination and then cultured in distilled water without TFIBA, the roots were elongated significantly by 40% only in the seedlings treated with 10−4 mol L−1 TFIBA. No inhibition of root hair formation was observed in these seedlings. Uptake of K+ and decreased drastically in the roots whose elongation was increased by 80% by a continuous supply of 10−5 mol L−1 TFIBA. In contrast, uptake of both ions increased significantly in the roots whose elongation was increased by 40% with a supply of TFIBA only during seed germination. These results strongly suggest that treatment with TFIBA during germination promotes root elongation without inhibiting root hair formation and increases ion uptake by seedlings.