The object of this study was to evaluate the effect of increasing amounts of dietary fish oil on growth and hematological variables of the weanling male Sprague-Dawley rat. Animals were fed diets containing either fish oil (FO) or sesame oil (SO) at 5, 10 or 15% (w/w) for 31 d. Growth retardation and reduced food intake was noted in groups fed FO. Hemoglobin (Hb) concentration diminished when the dietary FO was above 5% (w/w). FO is a poor source of (n-6) fatty acids. We postulate that a partial deficiency in (n-6) polyenic family, is a consequence of the increasing amounts of FO in the diets, that may affect growth and erytropoiesis. In this report we show evidence supporting the hypothesis that diets enriched with fish oil can alter normal growth and induced hematological changes in the male weanling rat.