It was found in a further analysis of the regeneration of the physiological tip in decapitated Avena coleoptiles, that both kinetin and gibberellic acid (GA3) completely antagonized the inhibition of the auxin synthesis in the new tip caused by 0.01 mg/l indoleacetic acid (I A A). To antagonize inhibitions caused by 0.02, 0.03 or 0.04 mg/l IAA, kinetin and GA3 had to be added in combination, but the effect of 0.05 mg/l IAA could no longer be neutralized by these hormones. Phenylbutyric acid did not affect tip regeneration. The results are briefly discussed in connection with the regulation of breaking apical dominance, and with the limited sensitivity of the Avena straight-growth test.