We reported that the growth promoting activity of di-n-butylphthalate (DBP) was observed when rats were fed with a niacin-free and tryptophan-limiting diet (Shibata et al., 1982. J Nutr Sci Vitaminol 28, 173-177). The present experiment was performed to in-vestigate whether this phenomenon is attributable to the increase in the conversion ratio of tryptophan to niacin. The weaning rats were fed with a 10% (low protein diet) or 20% pro-tein (conventional protein diet) diet with or without adding 1% DBP. The conversion ratio of tryptophan to niacin was significantly higher in the DBP group than in the control group; for 10% casein diets, it increased two-fold and for 20% casein diet, about five-fold. From these results, the previous finding is possibly explained by DBP increasing the conversion ratio of tryptophan to niacin.