Male Sprague Dawley rats were fed cholesterol-free or cholesterol-enriched purified diets containing 15% protein during 3 weeks. Three animal proteins and 6 plant proteins were used in the diets. Results indicated no specific effect of dietary proteins on serum cholesterol related to their animal or plant origin. Dietary proteins were also subjected to a pepsin-pancreatin in vitro digestion. Significant positive correlations were observed between serum cholesterol of rats fed cholesterol-enriched diets, and the tyrosine content of digestion products. This correlation was higher than that observed with intact dietary proteins. A following study showed that tyrosine supplementation may increase plasma cholesterol when diets contain a protein from which tyrosine is slowly released during enzymatic hydrolysis, indicating a relationship between the availability of dietary amino acids during digestion and plasma cholesterol. It has also been shown that this tyrosine effect was not mediated through plasma thyroid hormones.