In humans, diets containing high levels of soy isoflavones have shown to increase circulating estrogen levels. With this increase, early attainment of puberty has been experienced. Although a negative in humans, this could be a substantial gain in the some species of livestock. This research is designed to determine the effects of soy isoflavones on early development and early attainment of puberty in ewe lambs. Newly born Rambouillet and Suffolk ewe lambs were randomly assigned to either the control group; cotton seed meal protein, or the treatment group; soybean meal protein. Lambs were fed treatment specific diets from just following birth until breeding. Weights and blood samples were drawn to determine growth and performance as well as serum estrogen and progesterone levels. No difference (P > 0.05) in growth and performance and reproductive activity was established between treatments. However, a difference was established (P < 0.05) between estrogen concentration.