Abstract:Two rice ( Oryza sativa L. ) cultivars, tall cv. "Taizongyu 65" and dwarf cv."Jiangsu’ai", were used in the experiments to study the biological efficacy of pentefezol (S-07) to endogenous IAA in dee seedlings. When dee seeds were soaked in S-07 solution for a period of time, the rice seedlings were dwarfed as the content of endogenous IAA was reduced. The results indicated that S-07 enhanced the activity of IAA-oxidase and reduced the rate of transformation of 3H-tryptophan into 3H-IAA. Thus the endogenous IAA content of rice seedlings were reduced by S-07 treatment leading to subsequent retardation of the growth rate of seedlings. Moreover the rate of distribution of 3H-IAA in leaves and roots of rice seedlings were also reduced by S-07 treatment.